Quick answer: how to calculate a mitered border
Finished width equals top width plus 2 times border width, and finished length equals top length plus 2 times border width. Each mitered strip is longer than the side it borders by 2 times border width plus 2 times seam allowance, because the diagonal must run from the inner corner to the outer corner. Total strip length equals 2 times side strip length plus 2 times top and bottom strip length. Strips are rounded up, and yardage to buy is exact yardage rounded up to the nearest 1/8 yard.
Example: a 60 by 72 inch top, 4 inch border, seam 0.5 inch, usable fabric width 44 inches. Finished size is 68 by 80 inches. Side strips are 81 inches each, top and bottom strips are 69 inches each, total strip length is 300 inches, strips are 7, exact yardage is 0.78 yards, yardage to buy is 0.88 yards.
Allowance note, stated plainly: this calculator adds 2 times seam allowance to each strip, which is 1 inch with the default seam 0.5 inch. Some other sources advise a larger trimming margin, 2 to 4 inches, or 6 inches per strip, for trimming and for matching prints. That larger margin is a safety allowance, not part of the miter geometry, and this calculator does not add it. If you are a beginner, or you must match a stripe across the diagonal, buy an extra 1/8 yard, or cut strips a little longer and trim, instead of trusting the exact yardage.
If you only need a number, use the Mitered Border Calculator. If you want to understand why mitered strips are longer, and when to choose a miter over a straight border, read on.
What a mitered border is
A mitered border is a border where neighbouring strips meet at a 45 degree angle at each corner, instead of meeting at a square corner. Seen from the front, the corner looks like a diagonal line running from the inner corner of the border to the outer corner. It is the same idea as a mitered picture frame.
A mitered border is still part of the quilt top, added before backing, batting and quilting. It is not binding. Binding wraps the finished edge later, after quilting.
When a mitered border suits, and when it does not
| Choose mitered | Choose straight instead |
|---|---|
| Striped or directional fabric, where a continuous line around the corner looks best | Scrappy borders, where many fabrics meet at corners |
| A formal, framed look, with one clear border fabric | Borders with pieced corner squares as a design feature |
| A border you want to read as one frame, not four separate strips | A first attempt at borders, where square corners are more forgiving |
Why mitered strips are cut longer
Picture one corner. A straight side strip stops at the corner of the quilt top. A mitered side strip must reach past that corner, because its end is cut diagonally, and the diagonal seam must run from the inner corner to the outer corner of the border. That diagonal needs border width of extra length on each end.
On top of that, each miter end needs seam allowance for the diagonal seam itself. So each mitered strip is longer than the side it borders by 2 times border width plus 2 times seam allowance. Yardage uses cut width, which equals finished border width plus seam allowance on both sides, so with 0.5 inch seam allowance a 4 inch finished border is cut 5 inches wide. That is the whole secret of the calculation, and it is why straight border lengths must never be used for a mitered border.
Step by step: how to calculate a mitered border
Step 1: Measure the quilt top
Measure the finished top, pressed flat, in two places for width and two places for length. Use the larger number for each. Square up the top first. A mitered corner will show an uneven top more clearly than a straight corner.
Step 2: Add border width to get finished size
Finished width equals top width plus 2 times border width. Finished length equals top length plus 2 times border width. Border width is added on each side, so it is counted twice.
Step 3: Work out mitered strip cut lengths
Side strip cut length equals top length plus 2 times border width plus 2 times seam allowance. Top and bottom strip cut length equals top width plus 2 times border width plus 2 times seam allowance.
Step 4: Add all four strips and work out strips needed
Total strip length equals 2 times side strip cut length plus 2 times top and bottom strip cut length. Compare total length with fabric width. If it fits in one fabric width, you need 1 strip. If not, each extra strip adds fabric width minus seam allowance, because seam allowance is lost at each join. Round strips up.
Step 5: Convert strips to yardage
Fabric length to buy equals strips times cut width. Cut width equals finished border width plus seam allowance on both sides. Divide fabric length by 36 to get exact yardage in inch mode. Round up to the nearest 1/8 yard.
Try it with the calculator: Use the Mitered Border Calculator with your own top size, border width and fabric width. It uses the same steps as this guide, and it shows finished size, mitered strip lengths, strips needed and yardage together.
Worked example: a full quilt
Take a full top that measures 80 by 92 inches. Add a 4 inch mitered border, with 44 inch fabric and 0.5 inch seam allowance.
Finished size: width 80 plus 8, which is 88 inches. Length 92 plus 8, which is 100 inches.
Side mitered strips: 2 strips, each 92 plus 8 plus 1, which is 101 inches long. Top and bottom mitered strips: 2 strips, each 80 plus 8 plus 1, which is 89 inches long. Total strip length: 2 times 101 plus 2 times 89 equals 380 inches.
Strips needed: 380 inches does not fit in 44 inches. Each extra strip adds 43.5 inches. Strips equal 1 plus the ceiling of (380 minus 44) divided by 43.5, which is 9 strips.
Fabric length to buy: 9 times 4 equals 36 inches. Exact yardage: 36 divided by 36 equals 1.00 yard. Yardage to buy equals 1.00 yard.
With 108 inch fabric, the same border needs 4 strips, and yardage to buy is 0.50 yards. Wide fabric uses fewer strips, but check price per yard, not only total yards.
Mitered border table, our own computed table
This table uses a 60 by 80 inch top, usable fabric width 44 inches, and seam 0.5 inch, and it is computed from the same mitered strip rule as the calculator, not copied from another site. Cut width equals finished border width plus seam allowance on both sides, and yardage follows the calculator rule, strips times cut width, rounded up to 1/8 yard.
| Border width, finished | Finished size | Side and top strips | Total strip length | Strips | Yardage to buy |
|---|---|---|---|---|---|
| 2 in | 64 by 84 in | 85 in and 65 in | 300 in | 7 | 0.63 yards |
| 4 in | 68 by 88 in | 89 in and 69 in | 316 in | 8 | 1.13 yards |
| 6 in | 72 by 92 in | 93 in and 73 in | 332 in | 8 | 1.63 yards |
How to make sure your result is correct
- Finished size check. Finished width must equal top width plus 2 times border width, and finished length must equal top length plus 2 times border width.
- Strip length check. Each mitered strip must equal side length plus 2 times border width plus 2 times seam allowance. Straight border lengths will always come up short at a miter.
- Total and strips check. Total strip length must equal 2 times side strip length plus 2 times top and bottom strip length, and strips must be rounded up, never down.
- Buy versus exact check. Buy yardage must never be below exact yardage. If it is, you rounded the wrong way.
- Usable width check. Enter usable fabric width after removing selvages, not bolt width, or strip count can be too low near a boundary.
- Print matching check. If a stripe must match across the diagonal, the calculator yardage does not cover print matching. Buy extra for that job.
How to sew a miter, in brief
- Sew each border strip to the top, stopping one seam allowance short of each corner. Do not sew past the corner yet.
- Fold the top at a corner so the two neighbouring border strips lie right sides together, with their ends aligned.
- Mark and sew the diagonal seam from the inner corner point out to the outer edge of the border, at 45 degrees.
- Check the corner lies flat and the diagonal meets the inner corner cleanly, then trim excess fabric and press the seam open.
- Repeat at the other three corners, one at a time, and check each corner before moving on.
This is a summary, not a substitute for practice. If you have never sewn a miter, sew one corner on scrap fabric first, with the same border width, before cutting into your border fabric.
Common mistakes that spoil a miter
- Cutting straight lengths. Straight strips stop at the corner, so the diagonal seam has nothing to sew to, and the corner comes up short.
- Sewing past the corner. If border strips are sewn all the way past the corner, they cannot be folded together to form the miter.
- Marking the diagonal from the wrong point. The diagonal must start at the inner corner point, where border meets top, or the miter will not meet cleanly.
- Stretching the bias edge. The diagonal edge is cut on the bias, so it stretches easily. Handle it gently, and avoid pulling it while sewing.
- Matching a print at the miter without extra fabric. If you want a stripe or print to match across the diagonal, you will need extra fabric to shift the print into place. The calculator does not account for print matching, so buy extra for that job.
Tips for a clean miter
Press border strips before sewing, and press the top flat, so measurements stay true. Pin the diagonal seam carefully, and sew slowly across it, so the bias edge does not shift. Press miter seams open, so corners lie flat under quilting. If a corner does not meet perfectly, check the marking point first, before unpicking the whole border, because most miter problems start at the mark, not at the sewing machine.
Calculate your own mitered border now
Use your top size, border width and fabric width in the calculator, and check the result against the steps in this guide.
Open the Mitered Border CalculatorFrequently asked questions
It can, because mitered strips are longer. Yardage steps up in whole strips, though, so for some sizes the yardage is the same as a straight border, and for other sizes it needs one extra strip. Use the calculator above for your exact size, instead of guessing.
Yes, and stripes are one of the best reasons to miter, because the stripe can continue around the corner. If you want the stripe to match exactly across the diagonal, buy extra fabric, because matching a print needs room to shift the strip into place.
Check the marking point first. The diagonal must start at the inner corner point, where border meets top. Also check that border strips were stopped one seam allowance short of the corner, and that the bias edge was not stretched while sewing.
It needs more care than a straight border, but it is not out of reach. Practice one corner on scrap fabric first, with the same border width, and follow the steps in this guide one corner at a time.
Yes. Calculate each border separately, with its own method, and use the finished size from one border as the top size for the next. Do not use one method for both, or strip lengths will be wrong for one of them.
Related guides and calculators
Next, read How to Calculate Border for straight borders, and use the Border Calculator for square corners. For border fabric alone, see the Border Yardage Calculator, and for the finished edge, see the Binding Calculator.