Bodice Block Calculator
A basic block from three measurements.
Draft a basic bodice block from bust, waist and back length, with armscye depth, back and front widths and waist intake.
Quarter bust — block width
10.00"
40.00" bust with ease
These are drafting conventions rather than a standard, and every system uses slightly different ones. A drafted block gets the proportions close from measurements alone; posture, shoulder slope and body shape vary in ways three measurements cannot capture, so make it in calico and fit it before trusting it.
How the Bodice Block Calculator works
A bodice block is the foundation every other pattern is derived from: a close-fitting shell with no design, just the shape of the body plus the ease needed to move. The proportions here are drafting conventions rather than a standard — the armscye depth as a fraction of the bust, the shoulder slope — which is how every drafting system works, and why a block always needs fitting after it is drafted.
Also known as: sloper calculator · basic block calculator · bodice draft calculator
Conventions, not a standard
A bodice block is a close-fitting shell with no design features: the shape of the body plus the ease needed to move. Every other pattern is derived from it, which is why fitting the block once is worth more than fitting every pattern.
The proportions used to draft one — the armscye depth as an eighth of the bust plus a constant, the back width, the shoulder slope — are conventions. Different drafting systems use different ones and all produce a workable starting block, which is a clue about how much fitting still has to happen afterwards.
Two to four inches of ease at the bust is the usual range for a fitted woven block. Below two the garment cannot be moved in; above four it stops being a foundation and starts being a design.
A worked example
A 36 inch bust with four inches of ease gives 40 inches to work with, so a quarter bust of 10 inches — the width of each quarter of the block. The armscye depth is 40 ÷ 8 + 2.5, which is 7.5 inches.
With a 28 inch waist, the intake per quarter is (40 − 32) ÷ 4, which is 2 inches distributed between the side seam and the waist darts. Those figures give a block; they do not give a fit.
Where the answer misleads
Three measurements cannot capture posture. A rounded upper back, square or sloping shoulders, a forward neck — all change a block substantially and none appears in bust, waist and back length. This is why the block is made in calico and adjusted, every time.
Back width and shoulder length are the measurements most often wrong on a drafted block, because they vary independently of bust size and the formulas assume they do not. If a drafted block pulls across the back, that is usually why.
Finally, a block is for one fabric behaviour. A block fitted in calico works for stable wovens; knits and drapey fabrics need their own, with different ease, and using the woven block for both produces garments that are too big in one direction and too small in the other.
Frequently asked questions
How much ease should a bodice block have?
Two to four inches at the bust for a fitted woven block. Less than two and the garment cannot be moved in; more than four and it stops being a close-fitting foundation and starts being a design.
What is the difference between a block and a pattern?
A block is the fitted shell with no seam allowance and no design features. A pattern is a block with a design applied, seam allowances added and the pieces separated. You fit the block once and reuse it for everything.
Do I still need to fit the block?
Always. A drafted block gets the proportions close from measurements alone, but posture, shoulder slope and body shape vary in ways three measurements cannot capture. Make it in calico and adjust before trusting it.
Which measurements matter most?
Bust, waist and nape-to-waist do most of the work. Back width and shoulder are the ones most often wrong on a drafted block, because they vary independently of bust size and the drafting formulas assume they do not.