Quarter square triangles put the straight grain on the outside edge: that’s their whole job, and it costs 1¼ in of oversize. Hourglass blocks, QST borders, and setting fills all run on the same rule, worked here for your size and count. Cutting 60 degree equilateral triangles for a whole-top triangle quilt is strip math instead; this page covers the square-cut kind.
Your Triangles
Cut
20squares
5 ¼″ each · 10 pairings → 20 hourglasses
Pair A on B, sew both sides of one diagonal, cut: two big HSTs. Face them seam-on-seam, sew both sides of the crossing diagonal, cut: two hourglasses.
- Starting square5 ¼″ (4 + 1¼)
- Fabric A · Fabric B10 · 10
- Trim each unit to4 ½″
The Formula
starting square = finished + 1¼, cut on both diagonals → 4 quarter trianglesEach quarter triangle’s long side lies on the square’s edge, straight grain, so when it sits on a block or quilt edge, the edge doesn’t stretch. The two diagonal cuts cross four seam allowances, and 1¼ in pays for all of them.
Worked Example
Twenty 4 in hourglass units from two fabrics:
- Starting squares: 4 + 1¼ = 5¼ in.
- The HST shortcut: pair 5¼ squares like two-at-a-time HSTs, make two oversized HSTs, pair those facing, seam across their diagonal → two hourglasses per pairing. Twenty units = 10 pairings = 20 squares, ten of each fabric.
- Trim to 4½ with the ruler’s 45° on both seams.
QST Cut Chart
| Finished | Starting square (+1¼) | Oversize for trimming |
|---|---|---|
| 2″ | 3 ¼″ | 3 ⅝″ |
| 3″ | 4 ¼″ | 4 ⅝″ |
| 4″ | 5 ¼″ | 5 ⅝″ |
| 5″ | 6 ¼″ | 6 ⅝″ |
| 6″ | 7 ¼″ | 7 ⅝″ |
| 8″ | 9 ¼″ | 9 ⅝″ |
Keeping Hourglasses Crisp
- Press the first seams open or opposing, so the center crossing nests instead of stacking four allowances.
- Pin the center seam-to-seam before the final pass; a matched crossing is the whole look.
- Handle bias gently: the short edges are bias until the unit joins something. Starch helps slippery fabrics.
Assumptions and Defaults
- Hourglass counts use the two-per-pairing shortcut; four-fabric QSTs cut each fabric’s square separately and mix quarters.
- Loose QSTs (setting fills, border ends) come four from each square.
- Oversize column adds ⅜ for trimming: hourglass centers reward it more than any other unit.
Triangle Questions, Answered
What size squares do I cut for quarter square triangles?
Quarter square triangles start from a square at the finished size + 1¼ in, cut on both diagonals, so a 4 in unit starts from a 5¼ in square. That is ⅜ in more than the HST rule, because two seams cross each quarter instead of one.
Does my block need an HST or a QST?
To tell whether a block needs an HST or a QST, look at where the triangle’s long side ends up: on the unit’s outside edge, use a QST so the edge stays on the straight grain, whereas inside a seam, use an HST. Hourglass units are made of two QST quarters from each of two fabrics, with all the long sides facing out.
How do I make an hourglass block quickly?
The quickest way to make an hourglass block is the HST shortcut in the example: sew oversize HSTs first, then pair them and sew across, which gives two hourglasses per pairing from one drawn line. It is both faster and more accurate than sewing four loose triangles together.
Why is the center of my hourglass block lumpy?
An hourglass center is lumpy because four seam allowances stack at that point. Press the first seams open, or spin the final crossing, pin the middle, and trim the unit to size afterward, since the 1¼ in allowance leaves room to correct it.