How the calculators work
Last updated 2026-10-28
Area and volume
Rectangular areas are length times width. Wall area for a room is the perimeter times the wall height, then a standard door and window allowance is subtracted when the calculator asks for those counts. Volume is area times depth. Depth entered in millimetres is divided by 1,000 so the volume is in cubic metres.
Paint uses paintable area divided by coverage per litre, times the number of coats. Concrete, gravel, and sand turn that volume into weight with a density, then into bags. Sod and tile divide the area by the piece size.
Waste and rounding
A waste allowance is a multiplier: quantity times (1 + waste ÷ 100). It covers offcuts, spillage, and a small order buffer. The default is a starting point you can change. Straight tile layouts default lower than layouts with many cuts; diagonal and herringbone need more, and the field is there so you can raise it.
Bags, cans, rolls, sheets, and boxes always round up to a whole unit. We would rather you have one spare than stop mid-job. Volumes and areas stay rounded to two decimal places so you can still compare a bulk order.
Units
Each field shows its unit next to the input. Coverage, bag size, and density are editable because products differ. A paint can that says 10 m² per litre should be entered as 10, even if the default is different. The Dulux page below is an example of a labelled coverage, not a number every paint uses.
What we do not claim
Results are estimates for shopping, not a specification and not a substitute for local building code or a supplier’s takeoff. Soil, concrete, and paint vary. Measure twice and confirm the bag yield on the product you actually buy.
Sources
| Figure | Source | Checked |
|---|---|---|
| An 80 lb bag of QUIKRETE Concrete Mix yields about 0.60 cubic feet. About 45 bags make 1 cubic yard. | The Home Depot listing for QUIKRETE 80 lb. Concrete Mix, No. 110180 (yield stated on the product page) | 2026-10-24 |
| One cubic yard of mulch covers 324 square feet at a depth of 1 inch. | Clemson Cooperative Extension, HGIC 1604 Mulch (revised 20 June 2016) | 2026-10-24 |
| Dulux Easycare Kitchen Matt is labelled up to 13 m² per litre. Real coverage depends on the surface. | Dulux Easycare Kitchen Matt product page | 2026-10-24 |
| Romabio Classico Limewash covers 80–100 sq ft per litre for one coat. The calculator starts at 80 sq ft/L (7.43 m²/L). | Romabio Classico Limewash product page, coverage table | 2026-10-27 |
| A 50 lb bag of QUIKRETE Deck Mud yields about 0.41 cu ft (11.6 L). | The Home Depot listing for QUIKRETE 50 lb Deck Mud, No. 154855, including the QUIKRETE answer on yield | 2026-10-27 |
| An 80 lb bag of QUIKRETE Scratch and Brown Base Coat Stucco covers 1.8–2.2 m² at 3/8 in. The calculator starts at 1.8 m². | QUIKRETE Scratch and Brown Base Coat Stucco data sheet, product No. 1139 | 2026-10-27 |
| One bag of Owens Corning R-13 unfaced 15 in × 93 in batt (E51) covers 106.56 sq ft, about 9.9 m². | The Home Depot listing for Owens Corning R-13 Unfaced Insulation Batt E51 | 2026-10-27 |
| A 50 lb bag of MAPEI Kerabond covers 6.97–8.36 m² with a 1/4 in square-notch trowel. The calculator starts at 6.97 m². | MAPEI Kerabond technical data sheet, typical trowel coverage | 2026-10-27 |
| A 60 lb bag of QUIKRETE Concrete Mix yields about 0.45 cu ft (12.7 L). A 40 lb bag yields about 0.30 cu ft (8.5 L). | QUIKRETE Concrete Mix data sheet, product No. 1101, yield table | 2026-10-28 |
| An 80 lb bag of QUIKRETE Mortar Mix lays up to 37 standard bricks. | QUIKRETE Mortar Mix data sheet, product No. 1102 | 2026-10-27 |
Tile and flooring calculators start from a waste allowance you can edit (often 10% for a straight layout). That allowance is not taken from a single manufacturer sheet, so it is not listed as a cited figure. Add more for diagonal cuts.