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The Cutting Table · a question, worked through

Do I need to measure the slanted sides of a trapezoid or bay window cushion?

Yes, but not for the reason you think: the slanted sides are the check digit on your drawing. Here is how the workshop runs the numbers.

Answered by

The Pattern Engineer of Detroit · · Updated

Pinned by Nate

The quick answer

Yes, measure both slanted sides of a trapezoid cushion, because they can differ, but the workshop treats them as a check, not the master dimensions. The bottom length, top length, depth and any straight end pieces fix the slant. If your measured slant doesn't match the calculated one, another number is wrong, and you're asked to re-measure.

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Key facts

  • Measure both slanted sides separately; they can differ.
  • Bottom, top, depth and any straight ends are the master dimensions.
  • Slant = the diagonal of half the length difference and the depth minus the straight ends.
  • A 90 in bottom, 52 in top, 27 in depth and 6 in ends give a slant of about 28 in (71 cm).
  • A tape run along an old cushion reads 1 to 1.3 in (2.5 to 3.3 cm) long.
  • Mismatched slants mean re-measuring every edge.
  • If posts or moldings got in the way, a template is drawn for your approval.

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Send the numbers in inches: length × depth × thickness, plus any angle. The workshop checks them with you, because foam has no undo button. — Nate

Leave your details and the workshop will call you to confirm measurements, fabric, foam and piping. No prices, dates or payment on this page: it is a request, not an order.

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In inches: length × depth × thickness, plus any angles or cut corners.

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“Do I really need the slanted sides?”

Measure both, because the two sides of a real bay can differ. Then treat them as a check: bottom length, top length, depth and any straight end pieces fix the slant. Angles are just lengths in disguise, partner, and so are slants.

The trapezoid cushions hub has more shapes, and the trapezoid cushion calculator runs the arithmetic.

The check, run on a real bay seat

One bay seat from a past order: 90 in (229 cm) bottom, 52 in (132 cm) top, 27 in (69 cm) depth, 6 in (15 cm) straight ends.

  1. Halve the length difference

    (90 − 52) ÷ 2 = 19 in of lean per side.

  2. Take the straight ends off the depth

    27 − 6 = 21 in of rise.

  3. Find the diagonal

    The square root of (19² + 21²) is about 28.3 in: a slant of about 28 in (71 cm).

  4. Compare it with the tape

    A written 24 in slant can't close this shape, so every edge gets measured again.

“My two angled sides don't match. Now what?”

A 68 in (173 cm) front, 40 in (102 cm) back and 17.25 in (44 cm) depth give slants of about 22 1/4 in (57 cm), so a side written as 21 in flagged an error and a full re-measure.

Two habits cause these slips. A tape run along an old cushion follows its curve and reads long. And posts or moldings push the tape around them; say so, and a template is drawn for your approval. The trapezoid measuring pattern shows where to hold the tape.

On a trapezoid cushion, bottom, top and depth fix the slanted side, so a measured slant that disagrees means another number on the drawing is wrong.

Nate
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Buyer questions

  1. A common question

    Do I have to measure the slanted sides of my bay window seat?

    The workshop's answer

    Not strictly. The bottom, top, depth and any straight end pieces fix the slant, and it is calculated to check your numbers.

  2. A common question

    Are both slanted sides of a trapezoid cushion the same?

    The workshop's answer

    Not always, so measure both. Before cutting, front, back, depth and sides are checked to see that they close; if they don't, you're asked to re-measure every edge.

  3. A common question

    My bay seat has a hard maximum length. Can the cushion stay under it?

    The workshop's answer

    Yes. Handmade cushions can vary by about 1 in (2 to 3 cm) from the ordered size, so name the hard maximum and the cushion is kept under it.

Questions buyers ask next

Is the slanted side always longer than the seat depth?

Longer than the rise it spans, yes. Take any straight end pieces off the depth first; the slant is the long side of the right triangle that remains.

What are straight ends on a bay seat drawing?

Short square sections at each end of the seat, running front to back before the angle starts, like the 6 in (15 cm) ends above. Subtract them from the depth first.

Does the workshop cut my slant or the calculated one?

The shape follows the master numbers: bottom, top, depth and straight ends. The slant is calculated from them, and any mismatch is settled with you first.