Half Square Triangle Calculator
Quickly determine the square size to cut for half-square triangles (HSTs), how many squares you need, and a fabric yardage estimate for your quilt project.
Cut Square Size
5.125"
Finished + 0.875" seam + 0.25" trim
Squares Needed
10
Based on two at a time method
Fabric Estimate
Approximately 0.20 yards
(assumes WOF cutting efficiency)
How to Use This Calculator
Decide on finished size
Enter the finished measurement of one HST unit (after seam allowances are sewn).
Choose construction method
Select how many HSTs you create per pair of squares—two-at-a-time, four-at-a-time, or eight-at-a-time.
Review cutting size & quantity
Cut squares to the suggested size and prepare the number of squares shown to complete your blocks.
Formula
Cut Square = Finished Size + Method Allowance + Extra Trim
Squares Needed = ⌈Total HSTs × Method Multiplier⌉
Method allowances: Two-at-a-time = ⅞", Four-at-a-time = 1¼", Eight-at-a-time = 1¾"
Fabric estimate: Yardage ≈ (Squares × Cut Size²) ÷ 1296
Example: Finished 4", two-at-a-time → Cut = 4 + 0.875 + 0.25 = 5.125". 20 HSTs → 10 squares. Yardage ≈ (10 × 5.125²)/1296 ≈ 0.20 yd per color.
About the Half Square Triangle Calculator
Half-square triangles are a foundational quilting unit. Getting the math right ensures crisp points and enough fabric without waste. This calculator adapts to different construction methods so you can sew faster with confidence.
When to Use This Calculator
- Planning a quilt: Determine cutting dimensions before slicing fabric.
- Batch sewing: Estimate how many squares to stack for chain piecing.
- Workshops or classes: Provide students with quick size references.
- Pattern adaptation: Resize blocks from a pattern while keeping seam accuracy.
Why Use Our Calculator?
- ✅ Supports multiple methods: Choose two-, four-, or eight-at-a-time techniques.
- ✅ Custom trimming: Add extra allowance for perfect trimming.
- ✅ Fabric insight: Approximate yardage for fabric planning.
- ✅ Mobile friendly: Use in your sewing room or at the quilt shop.
Common Applications
Modern quilters: Resize blocks for custom layouts or sampler quilts.
Pattern designers: Provide precise cutting charts for patterns.
Guild swaps: Standardize units for block exchanges.
Tips for Best Results
- Add ⅛–¼" trimming allowance if you like to square units after sewing.
- Press seams open or to the dark side to reduce bulk.
- Use a marking tool to draw diagonal lines accurately before sewing.
- Test the method on scrap fabric to fine-tune seam allowance on your machine.
Frequently Asked Questions
Why add extra trimming allowance?
Adding ⅛–¼" lets you square HST units for perfect points, compensating for slight seam variations.
Do I need separate yardage for each color?
Yes. The yardage estimate is the area of all squares. Split between colors based on how many squares you cut from each fabric.
Can I use this for charm squares or layer cakes?
Absolutely. Enter the finished size you want and see whether pre-cut squares are large enough for the method you choose.
What about bias trim or triangle paper?
Triangle paper often includes trimming allowance. If using paper templates, set extra trim to zero and follow the template instructions.