The Studio · Calculator
Trapezoid cushion calculator
Every odd shape is just a list of honest measurements. Give me three of them and I will give you the rest, check your sketch to ± ¼ in and print the pattern at full size.
The quick answer
Three measurements decide a trapezoid cushion: front width, back width and the straight depth between them. For a symmetric shape each slanted side is √(((front − back) ÷ 2)² + depth²), and it is always longer than the depth. Enter any three values and the calculator returns the rest, checks your sketch and prints a 1:1 template on A4 or Letter sheets.
The trapezoid cushion calculator
Every field is live. Change a number and the drawing redraws; copy the link and it opens with your numbers.
Load an example — the ones marked are past projects
Drawing…
Buyer questions
- A common question
What happens if the slanted side measurements on my trapezoid sketch don't add up?
The workshop's answerThe bottom, top and depth always decide the slant, so the geometry is checked before cutting. With 70¾ in at the bottom, 39¼ in at the top and 21¼ in of depth the slanted sides must be about 26½ in, not the 25½ in written — so they were made at 26½.
- A common question
My sketch says 65¼ in at the top, 70¼ in at the bottom, 17¾ in high and 16¼ in slanted sides. Is that right?
The workshop's answerIt can't all be right: a slanted side is always longer than the straight height. If 16¼ in is correct, the height is about 16 in; if 17¾ in is correct, the sides are about 18 in. The buyer re-measured — 17¾ in and 18 in — and it went to production.
- A common question
Are both slanted sides of a trapezoid cushion the same?
The workshop's answerMeasure both; they can differ. A 68 in front, 40 in back and 17¼ in of depth give slants of about 22¼ in, so a 21 in side on the sketch flags an error. If posts or moldings forced you to measure around them, say so and a template is drawn for your approval.
- A common question
My window seat is narrower at the back and wider at the front. How do I order it?
The workshop's answerThat shape is a trapezoid. Choose the size by the longest edge and the deepest depth, then upload a drawing with every edge measured, plus the thickness and fabric. The cushion is made exactly to that drawing.
- A common question
Two seat trapezoids, 24½ in at the back, 27½ in at the front, 27¼ in deep, and a 73¼ / 78 × 27¼ in bench. What are the sides?
The workshop's answerWith tapers that slight the sides are only a little longer than the depth — about 27.2 in on the seats and 27.3 in on the bench — but they are the numbers the pattern is cut to. The buyer worked them out with a trapezoid calculator before ordering.
The dog, the linen and the corners that weren't square
A buyer with a dog chose washable linen for a long angled window seat and sent a sketch: 112 in along the long edge, 18 in deep, end sections of 32½ and 31 in, and slanted edges written as 35 in. The first photo was blurred; the second was sharp enough to draw to scale — and the drawing disagreed with the 35 in. The corners were not square.
The workshop did what I would do, partner: kept the edges that sit against walls as the masters — 112 in, 18 in, 32½ in and 31 in, which make the short edge about 48½ in — and let the diagonals fall where the geometry puts them. Small differences disappear into the foam and the sewing tolerance. The buyer confirmed, and it went into production.
The lesson: when a sketch argues with itself, decide which measurements touch walls. Those are the truth.
How to use it
Symmetric or not
Symmetric if both ends slant the same. A window seat with one square end and one angled end is asymmetric — choose that and tell the calculator about the left end.
Enter the three you trust
Usually front, back and straight depth. Depth is measured square to the front edge, never along the slant.
Check your sketch
Pick all four — check my sketch and type the slanted side as you measured it. The verdict says whether it closes within ± ¼ in (± 0.5 cm).
Read the diagonals
Measure corner to corner on the seat and compare with the table. That is your squareness check, and it catches most mistakes.
Print the template
Choose Letter or A4, add a seam allowance if you are cutting fabric, and print at 100 % / Actual size. Check the 4 in bar and the 100 mm square, overlap the gray bands, tape.
Lay it on the seat
Paper first, then foam, then fabric. If paper doesn't fit, nothing after it will.
SaveQuestions people ask
How do you calculate the slanted side of a trapezoid cushion?
Halve the difference between front and back, then use Pythagoras with the depth: side = √(((front − back) ÷ 2)² + depth²). For a 48 in front, 36 in back and 18 in depth: √(6² + 18²) = 18.97 in, about 19 in.
Why must the slanted side be longer than the depth?
Because the depth is the shortest distance between the front and back lines. Any slanted edge between the same two lines is longer; if your sketch says otherwise, one of the numbers is wrong.
How do I work out the corner angle of a trapezoid cushion?
The front-corner angle is the arctangent of depth ÷ set-back, where the set-back is (front − back) ÷ 2 for a symmetric shape. 18 in of depth and a 6 in set-back give 71.6°, which is 18.4° off square.
Can I print a trapezoid template at full size at home?
Yes. The template is split into Letter or A4 sheets with overlap bands and sheet names. Print at 100 % with “fit to page” switched off, check the calibration square, then tape the sheets together.
Is a trapezoid cushion made differently from a rectangle?
It takes extra cutting and sewing, so workshops usually price and produce it separately. More than one trapezoid has been ordered as a rectangle and had to be re-ordered with the drawing attached.
± 1/4 in — Nate