Fabric Calculator

A note from Jim Grant (Jan 27, 2017)

I am 72 76 years old now. This fabric calculator may be the last big project that I am able to carry out for Sailrite. It has required much of my time for the past three seven years. There has been no financial compensation for it. Like almost everything I have done for Sailrite, it was a labor of love. I was trained at the University of Chicago to be a teacher of political philosophy. I studied under Leo Strauss himself, as well as his students Joseph Cropsey, and Herbert Storing and later I taught with Martin Diamond and Harry Jaffa. The “Straussians” professed (and their students continue to profess) that serious study of the truth of things would generate a kind of satisfaction that the Greeks called “eros” or love. I don’t suggest that the love of my work approaches “eros”, but it at least gives me an inkling of what those very bright and wise old men were saying. Even though Sailrite has become a much more successful business enterprise than I could have imagined, let no one think that was my only goal. Nor should it be the only goal of the current work carried on by my sons Matt and Eric (or Jeff, Matt B, “Little” Matt, Brian, Hallie, Cassie and Chris who, while not my children, certainly perform similar functions in the business -- by the way, only Cassie in the list above has left Sailrite during the last 4 years and for good cause, to raise children -- I could add several more to the list of dedicated and very important people at Sailrite now). Connie and I were dedicated to proving that ordinary sailors could build and maintain their own sail and canvas inventories. So are these young men and women as are all the others taking orders and shipping goods. So don’t hesitate to ask questions and make suggestions. Sailrite’s goal is to provide clear and concise information and tools to make do-it-yourself satisfying and trouble free. It is with this in mind that I prepare the following documentation.

I must admit that this documentation has taken a back seat to the development of new calculators and to the creation of versions which will work as apps on Apple, Android, and Windows 10 devices. That work is still a work in progress. But the intent here should be to keep the free "web apps" uncomplicated and straight forward so that lengthy instructions are not required. To that end, I will spend the next few days attempting to clean up this document.


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This “calculator” started as a simple program to give yardage ordering information. But it has evolved into a more sophisticated tool which will not only calculate yardage but also show a rendering of the panels needed to complete a project. The renditions are very accurate but I make some assumptions for seams and hems that may not match your preferences. You should feel free to alter and change any patterns. Testing has been done for “normal” shapes and sizes to produce accurate patterns. Odd entries may result in outcome glitches. You should be diligent to review the patterns prior to cutting or ordering fabric.

Help for All Calculation Functions

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The app projects which require fitting now accept decimal input. Indeed, these apps, like those for box cushions and fold over cushions and wire hung canopies are intended to provide precise dimensions for all fabric parts that will guarantee a good fit.

Fabric Usage

The panel layouts prescribed by the software will not always be the absolute best in terms of efficiency. There will, for example, sometimes be scraps that can be used for additional boxing. And panels might be adjusted somewhat to reduce the yardage required a bit. But the graphic renditions should make these alternative layouts obvious and, in any case, they are likely to result in very minor savings with regard to the total yardage required for any given project. The fact of the matter is that simple algorithms cannot be perfectly efficient over a broad range of possibilities. They are, however, "close enough" to be quite useful.

Seam Allowances

Seam allowances can vary. Often the calculators will make it possible to indicate a preference. Or they will indicate an assumed width.

Flat Seams

When seams are required across a plate or across the width of boxing or for a wide awning, we add a 1 inch seam allowance. This is done in large part because most patterned fabric requires that overlap to make the pattern repeat consistent. The extra width also makes possible a felled seam (where folded edges are interlaid), but this felled seam is not required in a cushion or awning and it can be difficult to execute accurately. So keep it simple and just overlap these flat seams 1 inch (or trim to reduce seam width as desired).

The more recent trend is to provide for very narrow seam allowances (usually 3/8 inch). The two fabric layers are placed on top of one another and sewn with a straight stitch just that distance inside their matched edges. The intent is to provide for a stitch just far enough inside the raw edges to provide for good strength. Then the narrow seam allowances can be folded back each onto its own panel and pressed flat. This seam is not nearly so stiff as the sewn two layer assembly described in the paragraph above. It will be nearly invisible in the finished product.

Pattern from Existing Foam

We suggest that foam, where employed, be oversized. If it is used to pattern the fabric plates. This does not mean that you should slavishly follow all the inaccuracies in the foam (there may be some). Rather use the foam to mark corner points and join those points with straight lines where appropriate. If the foam edge is curved, mark offsets at one or two equal intervals and join the set of points with a smooth curve.

Pattern from a Surface

We suggest the use of light cardboard or clear pattern material to create templates. The template should match the expected finished size of the finished item with fabric covering. The template can be added to or subtracted from as necessary to create fabric and foam parts.

Box & Irregular Cushions

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Cushions sometimes have angled sides. The dimensions you enter should be derived by accommodating the angle. That is, measure the large (usually the top) side of the foam. There will be a little extra cloth allowed for the smaller plate in the final assembly. In addition, there may be curves along the outer edges of the foam. We allow nothing for this curve in the calculator for Box Cushions. If a curve exists, extra fabric should be ordered and additional spacing should be allowed between panels when using the computer renditions. For Irregular Cushions, with angled outer edges, there will likely also be a curved edge to match the shape of the hull. We allow up to 5% of the average of the length of the widest and narrowest ends for this curve in the calculator as open space between the drawn panels. If more is desired, extra fabric should be ordered and the rendered patterns must be adjusted accordingly. Forepeak cushions often have notches to allow for a head (toilet) or a bit of standing room. Include these cut away areas when measuring your cushions – the plates will be trimmed later to match the actual shape of the foam and the excess material can be discarded.

Seam Allowances

There is more than one way to build a cushion. Some prefer to add seam allowance on every edge. And some provide for no seam allowance at all. We prefer to make cushions following a few simple rules. The foam should be cut about ½ inch large all around the intended finished cushion size (assuming edge seams will be about ½ inch). The top and bottom plates of fabric should be cut to match the enlarged foam size. The boxing should be cut to the thickness of the foam plus about ½ inch. And the zipper plaque should be the thickness of the boxing plus the width of the closed zipper tape being used (this extra width is used for installation of the zipper). The latest version of the box cushion and fold over cushion calculator provide exact measurements for boxing and any required plaque. The irregular cushion calculator still requires integer data and renders boxing width oversized with no provision for plaque. It will be updated soon, but, in the meantime, some judgment will be required. The accompanying videos will guide that work.

Size Limits

The calculators are intended to function properly within a “normal” range. Outside this range, failure will be obvious so do not fear that you will obtain misleading results. When multiple panels are needed for individual plates, they will be numbered. For example panels A1 and A2 go together as do B1 and B2. If three panels are required for long striped cushions, the numbering will be A1, MID, and A2 (there will be no B panels here). Often there will be just three dimensions given for polygons with 4 unequal sides. This is justified since two measurements are for parallel sides separated by the third measurement at right angles to them. The missing dimension could easily be calculated with a little trig but it is not required for patterning. Note that numbered panels may have to be rotated to match up properly.

Partial Patterns

Some cushions are symmetrical and thus can be “reversed” – turned upside down to expose a fresh surface when needed. Such cushions are often covered with a single fabric. If this is desired, the Complete button (the default) should be used to calculate yardage. Piping will be assumed to run along both the top and the bottom of the boxing. But if there is no symmetry or if a less expensive, more durable, or more slide resistant fabric is desired for the underside, the Plate & Box or Plate Only buttons can be used to calculate the separate quantities of needed fabric. Plate & Box will give half the number of plates required plus all the boxing. Piping requirements in this case will be limited to just that required for the top seam along the boxing. The One Plate button will calculate fabric required for just half the number of plates (normally the bottom plates).

Box, Irregular, Folded Over & Platform Cushions

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Pattern Matching

Cushion covers are often “solid” in color (one color or a small scale pattern with no discernible pattern direction). If true for your fabric, use the “solid” button to calculate yardage. That will make the most efficient use of cloth possible. If there is an obvious direction to the pattern, note whether it runs the length of the roll of cloth (striped) or across the width (railroaded). Use the appropriate button to make the pattern run from side to side across the cushion and up and down along the boxing. Note the pattern on the drawing at the top of the calculator page – that is the normal way patterns are oriented on these cushions. But this is not an absolute rule. You may break it if you like.

Sail Shades

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Concave Sides

Sail Awnings are often attached only at the three corners or perhaps one corner and an opposite edge. Any "unattached" or "unsupported" length of fabric should be hollowed to prevent its dropping. The hollow is normally about 5% to 10% of the length of the edge deep at its center point. Note that the calculator shows all three sides as straight lines. The hollow should be trimmed away from the finished assembly prior to hemming/edge reinforcement.

Usage Tips

The sail awning calculator is useful for a very broad range of tasks. Any triangular shape such as the end wall of a tent or aft panel of an enclosure can be quickly broken down into a given number of panels of proper length and shape.


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Concave Sides

Tarpaulin are often attached only at the corners or perhaps at corners with a ridge pole. Any "unattached" or "unsupported" length of fabric should be hollowed to prevent its dropping. The hollow is normally about 5% to 10% of the length of the edge deep at its center point. Note that the calculator shows all sides as straight lines. The hollow should be trimmed away from the finished assembly prior to hemming.

Usage Tips

The tarpaulin calculator is useful for a very broad range of tasks. Any rectangular shape such as the walls of a tent or enclosure can be quickly broken down into a given number of panels of proper length.

Window Treatment

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Building a Relaxed Roman Shade

This shade will be lined. It will make use of two simple cord “lift lines” with rings. There will be one rib on the final segment. The edges will be finished with a double folded hem. We will build a shade to fit inside the frame of a window. That frame measures 33.25 inches (844.55 mm) in width and 45 inches (1143 mm) in height.

The first order of business in a project like this is to make good use of the fabric calculator. Hit the second tab below to pull up the calculator and enter width (32.75 inches in our example — there will be quarter inch clearance on each side) and height (45 inches in our example) and fabric width (54 inches here).

Note that the edge allowance defaults to 1.5 inches (38.1 mm).

Hit “calculate”. Scroll upward on the device to reveal the entire shade rendition.

Several things should be noted. First, rings are indicated only on alternate segments. Folds will be created between ring lines. They are formed by symmetrical segment pairs collapsing upon one another.

Second, the length and width dimensions are in green on the rendering. Edge allowance is in red and head and foot allowance is indicated with purple.

Third, there is only one rib in a relaxed shade in the form of a weight bar. It is placed at the top of the first permanent fold. It will be inserted in a tubular webbing sleeve that will be secured only at the sides of the shade. In addition to weighing down the bottom edge of the shade, this bar will draw the flair at the bottom of the shade in to the intended width and, thus, create the desired swag curve.

Fourth, lining fabric is included in the list of materials but it is optional. We will be using a lining with this shade.

Fifth, the equivalent of six extra segments is added to the length of the shade. This extra material will become three permanent folds at the bottom of the shade.


Relaxed: 1 — Create the Shade Panels

Cut the single 71.5 by 35.75 inch panel (1816.1 by 908.05 mm) that will make the face panel for this shade. Carefully note which end of your cut panel is “top” and which is “bottom” (some patterns do not look good upside down). Also cut lining fabric. It should be the width of the finished shade. In length it should match the length of the facing less 3 inches (76.2 mm).

Both the facing and the lining will be flared at the bottom – this flare is what creates the relaxed “swag”. The flare will start 6 segment depths above the bottom of the shade. The wider the flare is, the more “swag” there will be at the bottom of the shade. The width of the flair is governed by the percentage dropdown choice entered just to the right of the shade style selector. That percentage is the depth of the swag relative to the width of the shade.

Relaxed: 2 — Secure the Lining to the Facing

Place the facing, the decorative fabric, face up. Spread the lining (right side down) over it. Sew the lining in place first along one edge with the two layers and the bottom flush. Then smooth the fabric layers together over to the other edge until they are flush and sew. Be careful not to skew the lining as it is attached. Both stitches should be accurately placed 3/4 inch (19.05 mm) inside the edges.

Center the lining over the facing fabric. Iron the lining edges flat – that will crease the facing under the lining edges. Then fold the facing up over onto the back of the shade evenly on both sides and iron that nicely in place. This hem can be sewn in place with carefully matched thread or a clear nylon. Or the hem can just be secured with double-sided “Seamstick” tape. The shade width should now be as intended. Check it out. Relaxed: 3 — Finish the Shade Bottom and Turn it Right Side Out The shade should be face down and still inside out. Sew across the bottom with a straight stitch .75 inch (19 mm) inside the matched bottom edges. If there were no lining, the bottom edge would be finished by folding it over against the back of the shade .375 inch twice.

Then turn the shade assembly right side out. Poke the corners our carefully.

Relaxed: 4 — Sew the Rings in Place

Sew 2 rings just 1 inch (25.4 mm) inside the edges of the shade on the backside at the bottom of each segment pair. The first two will be right at the bottom of the shade. This can be done with a zigzag machine set at 0 stitch length or by hand with needle and thread. Use a thread similar in color to the shade fabric (or a clear nylon) and keep the length of the stitch on the face of the shade to a minimum in order to reduce its exposure.

The vertical spacing of the next set of 2 rings will be 2 “standard segments” higher and the next 2 more above that. The calculator indicates proper vertical spacing for these rings.

Use the same vertical spacing for rings across the shade from the flair line all the way to the top as indicated in the rendition.

Relaxed: 6 — Sew the Weight Rod in Place

Cut a brass weight bar with a hack saw so that it is .5 inches (12 mm) shorter than the finished shade width. Insert it into a length of tubular half inch webbing just long enough to fully enclose it. Leave an extra half inch or so at each end to sew it in place. Close the ends of the pocket with needle and thread and, at the same time, hand securing it to the outer edges of the shade just above the second set of rings above the bottom of the shade. It will be shorter than the shade at that point. The excess fabric between the ends of the weight bar will drop down to form the desired swag.

Relaxed: 5 — Install Lift Lines

Insert 2 lengths of the leech line recommended by the calculator (any small braided cord will do), each roughly the length of the full shade plus its width through the bottom 4 rings on each side of the shade. Tie these cords to the bottom rings. Then slide plastic orbs over the free ends and down to the 4th rings. These orbs are spring loaded to lock onto the cord while providing some measure of adjustability. Then thread each cord up through the rings above.

Wider shades will also have lift lines through the center that are secured (usually with the spring loaded orbs to make them adjustable) to the center rings at the flair line and run up to the top of the shade.

Relaxed: 6 — Headboard Installation

Prepare a headboard the as long as the width of the finished shade. The headboard can be wrapped like a present with scrap cloth stapled in place if desired. Or it can be painted. Put screw eyes in the bottom of the headboard that match the placement of the rings on the ribs. Accuracy here is important – otherwise the shade will be pulled to one side or the other when fully retracted. Install a small cord lock just 1 inch or so (25.4 mm) inside the right (or left) screw eye. Then staple Velcro tape (hook or loop) on the front of the headboard. Mount the headboard either inside or outside the window frame as intended.

Relaxed: 7 — Finish the Top of the Shade

Test fit the shade. There should be two inches of extra fabric at the top. Create a hem with this material folding it onto the back of the shade 1 inch (25.4 mm) and then make a second fold over this first one. Press the folds neatly with an iron making sure that the top is perpendicular to the sides. Baste and sew the mating Velcro tape to the top of the shade. Thread the 2 lift lines through appropriate screw eyes and on to the cord lock (it will be helpful to remove the cord lock in order to guide the lines in place). When all is threaded, the Velcro tapes can be smoothed in place over one another along the headboard.

Reduce the number of cord ends by using a condenser and a tassel if desired.