Calculate how much fabric to buy for bias binding from your quilt size and strip width, with the continuous bias starting square and tape yield.
Bias Binding Fabric Formula
To find the size of the starting square needed to make a given length of continuous bias binding, use the following formula:
S = sqrt(L * W * (1 + m/100))
To find how much fabric to cut off the bolt instead of cutting a square, divide the same fabric area by the usable width of the fabric:
F = L * W * (1 + m/100) / WOF
To work in the other direction and find how much binding a piece of fabric you already own will make, use:
Y = FW * FL / (W * (1 + m/100))
To find the strip cut width from a desired finished tape size, use C = 2 * f for single fold bias tape, C = 4 * f for double fold bias tape, and the following for quilt binding that is folded double before it is attached:
C = 4 * f + 2 * SA + t
Variables:
- S is the side length of the starting square of fabric
- L is the total length of bias binding needed
- W is the cut width of each bias strip
- m is an added-area planning allowance in percent (default 10%); it is not the percentage of fabric lost
- F is the length of fabric to cut off the bolt
- WOF is the measured usable width of fabric on the bolt (42 inches is an illustrative default)
- FW and FL are the width and length of a fabric piece you already own
- Y is the length of binding that piece will yield
- C is the strip cut width, f is the finished width, SA is the seam allowance, and t is a thickness allowance for the quilt sandwich
- All lengths use the same unit, either inches or centimeters
The calculator offers fabric-needed, project-perimeter, yield, and tape-width options. The default mode works out the binding length from your quilt or project dimensions as the perimeter plus extra for corners and joining, or takes the binding length you need, then returns the fabric to buy, the starting square size, the equivalent cut off the bolt, and how far a fat quarter would get you. The second mode reverses the math and tells you how much binding a fabric piece you already have will produce. The third mode converts a finished tape or binding width into the strip cut width, and points you to the closest standard bias tape maker size.
Bias Tape Maker Sizes and Starting Square Reference
Bias tape makers are labeled by the width of the single fold tape they produce, and a common planning rule uses strips cut to approximately twice that width. Follow the actual tool instructions, which can vary with fabric thickness. The table below connects the standard maker sizes to the strip width you should cut and the tape each produces.
| Tape maker size | Approximate cut strip width | Single fold tape | Double fold tape |
|---|---|---|---|
| 6 mm (1/4 in) | 1/2 in (1.2 cm) | 1/4 in | 1/8 in |
| 12 mm (1/2 in) | 1 in (2.5 cm) | 1/2 in | 1/4 in |
| 18 mm (3/4 in) | 1 1/2 in (3.8 cm) | 3/4 in | 3/8 in |
| 25 mm (1 in) | 2 in (5 cm) | 1 in | 1/2 in |
| 50 mm (2 in) | 4 in (10 cm) | 2 in | 1 in |
The next table shows the starting square needed to bind common quilt sizes, using the perimeter plus 12 inches of extra length and a 10% seam and waste allowance.
| Quilt size | Binding length | Square for 2 1/4 in strips | Square for 2 1/2 in strips |
|---|---|---|---|
| Crib (36 x 52 in) | 188 in | 22 in | 23 in |
| Lap (54 x 65 in) | 250 in | 25 in | 27 in |
| Twin (70 x 90 in) | 332 in | 29 in | 31 in |
| Queen (90 x 95 in) | 382 in | 31 in | 33 in |
| King (110 x 108 in) | 448 in | 34 in | 36 in |
Example Problems
Example 1: fabric needed for an apron and placemat set. You need 160 inches of bias binding cut 2.5 inches wide. The fabric area required is 160 * 2.5 * 1.10 = 440 square inches. The square root of 440 is about 20.98, giving a planning size of 21 x 21 inches. The area-equivalent bolt cut is 440 / 42 ≈ 10.48 inches, or 3/8 yard when buying in eighth-yard increments. Confirm the cutting layout and actual seam losses before cutting.
Example 2: binding from a fat quarter. You have an 18 x 22 inch fat quarter and want 2.5 inch strips. The theoretical maximum is 18 * 22 / 2.5 = 158.4 inches. Reversing a 10% added-area allowance gives 158.4 / 1.10 = 144 inches of estimated binding. This is an area-based plan; joining seams and trimmed corner strips affect actual yield. That is plenty for placemats or a garment edge, but short of the 188 inches a crib quilt needs, so plan on two fat quarters or a 23-inch square for that project.
Bias Binding Fabric Calculator FAQs
Why cut binding on the bias instead of on the straight grain? Strips cut at 45 degrees to the selvage stretch, which lets the binding curve smoothly around scalloped edges, necklines, and rounded corners without puckering. Bias binding also wears better on quilts, because the threads cross the edge at an angle and the abrasion is spread across many threads instead of running along a single one. For a straight-sided quilt, straight grain binding works and uses slightly less fabric, so bias is a choice, not a requirement.
Do you need to add seam allowance to the strip cut width? No. The strip cut width you enter is the full width of the strip as cut, and the added-area allowance is a planning margin for seams and trimming, not a guarantee that it covers every cutting method. What the cut width must already include is the folding structure of the tape itself: a 1/2 inch double fold tape needs a 2-inch strip, and a French quilt binding with a 3/8 inch finished width and 1/4 inch seams needs a strip around 2 1/8 to 2 1/4 inches wide. The quilt-binding option uses the method-specific planning rule 4 × finished width + 2 × seam allowance + thickness extra. Other French-binding methods differ; follow your pattern and test a sample.
How much bias binding does a fat quarter make? Divide the fat quarter area, 18 x 22 = 396 square inches, by your strip width, then divide by 1.10 for the default 10% added-area allowance. That gives 160 inches of 2 1/4 inch binding or 144 inches of 2 1/2 inch binding per fat quarter, before method-specific losses. Two fat quarters will bind most lap quilts, and the yield mode of the calculator will confirm the number for any piece in your stash.
