Sewing & Quilting HST (Half-Square Triangle) Calculator
Created by: Emma Collins
Last updated:
Plan your half-square triangle cutting with confidence. This calculator gives finished-to-cut size conversion, method-specific formulas, square count, and fabric estimate for HST-heavy quilts.
HST (Half-Square Triangle) Calculator
SewingGet cut size, unit count, and fabric estimate for precise HST piecing
HST Targets
Construction Preferences
Related Calculators
What is an HST (Half-Square Triangle) Calculator?
An HST calculator helps quilters determine the correct cut size for making half-square triangles. Enter your target finished HST size and quantity, then get cut measurements, unfinished size, required square count, and fabric estimates.
This tool supports both exact cutting and trim-friendly methods, making it useful for quick patchwork and precision piecing workflows.
HST Formulas
Unfinished HST Size: Finished Size + 1/2 inch
Exact Method Cut Size: Finished Size + 7/8 inch
Trim Method Cut Size: Finished Size + 1 inch
Square Pairs Needed: ceil(HST Quantity ÷ 2)
Fabric Area: (Squares Needed × Cut Size²) × (1 + Waste%)
Example Calculation
For a 3-inch finished HST, quantity 40, using trim method with 10% waste:
- Unfinished size: 3 + 0.5 = 3.5 inches
- Cut square size: 3 + 1 = 4 inches
- Pairs needed: 40 ÷ 2 = 20 pairs
- Squares to cut: 40 total
- Estimated fabric includes 10% overage for trimming and handling
Common Applications
- Patchwork planning: Determine cut sizes before cutting sessions
- Fabric shopping: Estimate extra fabric for large HST-heavy quilts
- Pattern conversion: Adapt block sizes while preserving finished dimensions
- Workshop prep: Pre-cut class kits with consistent triangle units
Tips for Accurate HSTs
- Use a scant quarter-inch seam to avoid undersized units
- Press, do not iron, to prevent bias stretch
- Trim to unfinished size for precision piecing
- Chain-piece pairs to save time and improve consistency
Frequently Asked Questions
What size square do I cut for HSTs?
For the two-at-a-time exact method, cut squares at Finished Size + 7/8 inch. For trim-friendly construction, cut squares at Finished Size + 1 inch and trim down after pressing.
What is the finished vs unfinished HST size?
Quilt blocks are usually discussed by finished size. The unfinished size is finished + 1/2 inch to include seam allowance. Example: a 3-inch finished HST is 3.5 inches unfinished.
Should I use exact or trim method?
Exact cutting is efficient and faster when your sewing is very accurate. Trim method is more forgiving and preferred for precision piecing, especially for many small HSTs.
How many HSTs do I get per pair of squares?
The standard two-at-a-time method yields 2 HST units per pair of equally sized squares.
How much extra fabric should I add?
A 5-10% waste allowance is common. Use 10-15% for scrappy quilts, directional prints, or if you are trimming all units.
Can I use this for metric measurements?
This calculator uses inches for quilting formulas. If you work in metric, convert your target finished size to inches first, then convert results back to centimeters.
Does pressing direction affect HST size?
Yes. Heavy steam and aggressive pressing can distort bias edges. Press gently and consistently, and trim if needed for accurate final units.
Sources and References
- Alex Anderson, The Essential Triangle Tool, C&T Publishing
- The Quilters Academy Vol. 1, Harriet Hargrave
- American Patchwork & Quilting, Precision Piecing Guidelines, 2024