Quick answer: how to calculate continuous bias binding
Square side needed equals the square root of needed bias length times binding strip width. Round square side up to the nearest 0.5 inch. Continuous bias length from a square equals square side squared divided by binding strip width. Yardage to buy is square side divided by 36, rounded up to the nearest 1/8 yard, if square side fits in fabric width.
Example: needed bias length 443 inches, strip width 2.5 inches. Square side needed is 33.28 inches, rounded up to 33.5 inches. Yield is 448.9 inches, which covers 443 inches. Yardage to buy is 1.00 yard.
Allowance note, stated plainly: this calculator now has an optional waste allowance, with 10 percent as the default, and 0 percent, 5 percent, or a custom percent as choices. Waste is added to needed length before the square is worked out, and the base square without waste is shown beside the final square, so the same extra is never counted twice. Its other safety is rounding square side up to 0.5 inch, which gives a little extra yield. Some other sources multiply yield by 0.9, or advise 10 to 15 percent extra. If your needed length is very close to the yield, use the next 0.5 inch square, instead of trusting the exact yield.
If you only need a number, use the Continuous Bias Binding Calculator. If you want to understand why the square root works, and when continuous bias is wrong, read on.
What continuous bias binding is
Continuous bias binding is bias binding cut as one long continuous strip, from a single fabric square. The square is marked into strips, sewn into a tube, offset by one strip, and then cut in a spiral. The result is one long bias strip, instead of many short bias strips joined end to end.
Continuous bias is still bias binding. It is cut on the diagonal, so it stretches around curves, just like regular bias binding. The difference is in how the strip is made, and in how many joins it has, not in the job it does at the quilt edge.
Why the square root formula works
The formula looks surprising at first, but it comes from area. A continuous bias strip is a long rectangle, with length L and width W, so its area is L times W. That strip comes from a fabric square, with side S and area S times S. If the strip area must equal the square area, then S times S equals L times W, and S equals the square root of L times W.
That is why the calculator uses a square root. It is not a guess, and it is not a table lookup. It is area, rearranged. In practice, seam allowances at tube joins and cutting take a little area, so the calculator rounds square side up to the nearest 0.5 inch, to cover that variation.
Continuous bias compared with regular bias
| Continuous bias | Regular bias |
|---|---|
| One long strip, far fewer joins | Many short strips, joined end to end |
| Needs a fabric square of calculated side length | Can be cut from yardage, in strips |
| Best for long bindings and curved edges, where joins would show or bulk up | Best when fabric is yardage, not a square, or when only a short bias length is needed |
| Yardage may be a little higher, because you must buy a full square | Yardage follows strip count, and may be a little lower for the same length |
Step by step: how to calculate continuous bias binding
Step 1: Get needed bias length
Needed bias length is edge length plus extra for joins, curves and overlap. For a rectangle, edge length is 2 times width plus 2 times length. For a curved quilt, measure around the edge directly, with a flexible tape, without stretching the quilt. Use finished size or measured edge length, after borders are added.
Step 2: Work out square side needed
Square side needed equals the square root of needed bias length times binding strip width. For example, if needed length is 443 inches and strip width is 2.5 inches, square side needed is the square root of 443 times 2.5, which is 33.28 inches.
Step 3: Round square side up
Round square side up to the nearest 0.5 inch, for practical cutting. In the example, 33.28 inches rounds up to 33.5 inches. Rounding up gives a little extra bias length, which is safer than rounding down.
Step 4: Check bias length you will get
Continuous bias length from a square equals square side squared divided by binding strip width. In the example, 33.5 squared divided by 2.5 equals 448.9 inches, which covers needed length of 443 inches. If the yield does not cover needed length, use a larger square.
Step 5: Work out yardage
If square side fits in fabric width, fabric length to buy equals square side, because you need one square piece. Exact yardage equals fabric length divided by 36 in inch mode. Yardage to buy is exact yardage rounded up to the nearest 1/8 yard. If square side exceeds fabric width, a single square cannot be cut from that fabric width. Piece a square, choose a wider fabric, or use regular bias binding instead.
Try it with the calculator: Use the Continuous Bias Binding Calculator with your own needed length or square size, binding strip width and fabric width. It uses the same steps as this guide, and it shows square size needed, continuous bias length, and yardage together.
Worked example: a queen quilt
Take a queen quilt with needed bias length 443 inches, binding strip width 2.5 inches, fabric width 44 inches.
Square side needed: square root of 443 times 2.5 equals square root of 1107.5, which is 33.28 inches. Rounded up to the nearest 0.5 inch, square side needed is 33.5 inches.
Continuous bias length you get: 33.5 squared divided by 2.5 equals 1122.25 divided by 2.5, which is 448.9 inches. This covers needed length of 443 inches.
Fabric length to buy: square side 33.5 inches, which fits in fabric width 44 inches, so fabric length to buy is 33.5 inches. Exact yardage: 33.5 divided by 36 equals 0.93 yards. Yardage to buy, rounded up to the nearest 1/8 yard, equals 1.00 yard.
Reverse check: a 33.5 inch square, with strip width 2.5 inches, gives 448.9 inches of continuous bias, which matches the forward calculation. With the default 10 percent waste allowance, needed length becomes 487.3 inches, square side becomes 35.0 inches, continuous bias length becomes 490.0 inches, and yardage to buy stays 1.00 yard in this example. The base square without waste, 33.5 inches, is still shown in the calculator result.
Continuous bias yield table, our own computed table
This table uses strip width 2.5 inches, and it is computed from square side squared divided by strip width, not copied from another site. Yardage to buy is square side divided by 36, rounded up to the nearest 1/8 yard, and it assumes square side fits in fabric width. If your square does not fit, piece a square, choose wider fabric, or use regular bias binding.
| Square side | Continuous bias yield | Yardage to buy |
|---|---|---|
| 18 in | 129.6 in | 0.50 yards |
| 24 in | 230.4 in | 0.75 yards |
| 30 in | 360.0 in | 0.88 yards |
| 36 in | 518.4 in | 1.00 yard |
How to make sure your result is correct
- Area check. Square side squared must be at least needed length times strip width, after rounding up, or yield will fall short.
- Yield check. Yield must equal square side squared divided by strip width, and yield must cover needed length. If it does not, use a larger square.
- Rounding check. Square side must be rounded up to 0.5 inch, never down, and yardage must be rounded up to 1/8 yard, never down.
- Fabric width check. Square side must fit in fabric width, or a single square cannot be cut. The calculator warns in that case, instead of giving a misleading yardage.
- Square check. The piece must be a square, not a rectangle, or the square root formula does not apply.
- Close call check. If yield is only just above needed length, use the next 0.5 inch square, because tube seams take a little area.
The tube method, in brief
- Cut a fabric square of the calculated side length, and press it flat.
- Mark the square into strips of binding strip width, parallel to the diagonal, following the continuous bias method you are using.
- Sew the square into a tube, offset by one strip width, so the marked lines form one continuous spiral, instead of separate loops.
- Cut along the marked lines in a spiral, starting at one end of the tube, to get one long continuous bias strip.
- Press the strip, handling bias gently, without stretching it, before sewing it to the quilt edge.
This is a summary, not a full continuous bias tutorial. If you have never used the tube method, practice on a small scrap square first, with the same strip width you plan to use, and check that the cut gives one continuous strip, not separate loops, before cutting into your binding fabric.
Common mistakes with continuous bias
- Rounding square side down. Rounding down gives bias length that does not cover needed length. Always round square side up.
- Forgetting to offset the tube by one strip. Without the offset, cutting gives separate loops, not one continuous strip. Check the offset before cutting.
- Using a rectangle instead of a square. The square root formula assumes a square. A rectangular piece gives a different yield. Piece a square, or use regular bias binding, if your fabric is rectangular yardage.
- Ignoring fabric width. If square side exceeds fabric width, a single square cannot be cut from that fabric. The calculator warns you in that case, instead of giving a misleading yardage.
- Stretching bias while cutting or sewing. Bias stretches easily, so a stretched strip gives a wavy edge. Handle bias gently, and ease around curves instead of pulling.
Calculate your own continuous bias binding now
Use your needed length or square size, binding strip width and fabric width in the calculator, and check the result against the steps in this guide.
Open the Continuous Bias Binding CalculatorFrequently asked questions
The standard continuous bias method needs a square, because the square root formula assumes square area. A rectangular piece gives a different yield, and it may not form one continuous spiral in the same way. Piece a square from your fabric, or use regular bias binding, if your fabric is rectangular yardage.
Yes, a little, because tube seams take a small amount of fabric. The calculator rounds square side up to the nearest 0.5 inch to cover that variation, but actual yield may vary a little with seam allowance and cutting. If you are close to needed length, use a slightly larger square.
Usually because the tube was not offset by one strip width before cutting. Without the offset, each marked line forms a separate loop. Check the offset step before cutting, and practice on a scrap square first.
Not always. Continuous bias is best for long bindings and curved edges, where fewer joins matter. Regular bias is simpler when fabric is yardage, when only a short length is needed, or when a square of the right size does not fit in fabric width.
Yes, continuous bias works on straight edges too, but it is most useful for curved edges, where bias stretch matters, and for long bindings, where fewer joins matter. For a short straight edge, regular straight grain binding is simpler.
Related guides and calculators
Next, read How to Calculate Bias Binding for regular bias strips, and Bias vs. Straight-Grain Binding to compare bias and straight grain. Use the Continuous Bias Binding Calculator for your own measurements, and for straight edges, see the Binding Calculator.