Why you buy more than you use
Two separate things push the purchase above the measured area: an allowance for cuts and loss, and rounding up to whole packages. The planner shows both so you can see which one is costing you.
Step 1: allowance
The measured net area is what will be installed. Cutting sheets to fit, trimming planks at walls, roller and tray loss — these consume material that never ends up on the surface. An allowance scales the area up: basis = net × (1 + allowance %). It is a planning assumption, prefilled at 10% for drywall and flooring, that you should review for your room's complexity. It is never a guarantee.
Step 2: package rounding
You cannot buy 14.8 sheets. packages = ⌈ basis ÷ coverage per package ⌉. The gap between the basis and the purchased coverage is leftover you did not choose.
Worked line
| Net drywall area | 431 sq ft | what gets installed |
|---|---|---|
| Allowance 10% | +43.1 sq ft | cuts and offcuts |
| Purchase basis | 474.1 sq ft | what you need to buy, in area |
| ÷ 32 sq ft per sheet | 14.82 | fractional sheets |
| Round up | 15 sheets | purchased coverage 480 sq ft |
| Leftover | 49 sq ft | 480 − 431, part allowance, part rounding |
Paint is different
Paint has coats and a loss allowance but no cutting. volume = net × coats ÷ coverage × (1 + loss %), then round up to cans. Do not reuse the drywall allowance for paint — paint covers the installed surface, not the offcuts.
Pooling can save a package
Two rooms each needing 3.125 sheets round to 4 + 4 = 8 if bought separately, but 6.25 → 7 if pooled. Pooling only makes sense for the same product and when you are happy to carry offcuts between rooms. The planner offers both and shows the per-room breakdown either way.
Mixed container sizes
Buying one gallon plus one quart instead of two gallons is a cost decision that depends on prices and how much leftover you want. The planner asks you to choose one container size and shows the leftover; it does not claim an optimal mix.