This page explains the rules every calculator on Modeplot follows. Each calculator’s own page shows its formula, a worked example and its assumptions.
Units
- All calculations run in SI units internally (metres, square metres, cubic metres, kilograms). Inputs are converted on entry and results converted for display, so mixing feet, inches and metres gives the same answer.
- Conversion factors are the exact definitions published by NIST: 1 inch = 2.54 cm, 1 foot = 12 inches, 1 cubic yard = 27 cubic feet, 1 US gallon = 231 cubic inches, 1 pound = 0.45359237 kg, 1 short ton = 2,000 lb.
- Length fields accept decimals, fractions (6 1/2) and feet-and-inches (12 ft 6 in, 12’ 6”).
- Switching between US and metric units keeps the physical size you entered: 12 ft becomes 3.6576 m, not 12 m.
Rounding
Intermediate values are never rounded. Rounding happens once, at the end:
- Whole items you have to buy – tiles, boxes, bags, sheets, bundles, boards – are rounded up.
- Displayed measurements are rounded for reading only; the next step still uses the exact value.
Waste allowance
Waste is entered as a percentage and applied as Quantity × (1 + Waste ÷ 100). The calculators show the quantity with and without waste. A default is pre-filled only where a published source supports it (for example, the Brick Industry Association’s minimum of 5 % for brick). Elsewhere the default is 0 % and the page explains what the allowance depends on.
Material properties
Coverage, bag yield, bundles per square, density and package sizes differ between products. The calculators ask you for the figure on your product’s label or data sheet. Where a calculator pre-fills an example value, it is listed below with its source; edit it to match your product.
Costs
Costs are only calculated from a price you enter. The site never supplies material prices.
What the calculators do not do
They do not design structures, size footings, beams or rafters, check building-code compliance, or recommend thicknesses or spacings. Those decisions need a qualified professional.
Testing
Every calculator has automated tests built from documented examples. The worked example on each calculator page is generated from the same test case, so the page cannot show a number the tests do not confirm.
Source register
| Source | What it supports | Checked |
|---|---|---|
| NIST Handbook 44, Appendix C – General Tables of Units of Measurement – National Institute of Standards and Technology | Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg. | 2026-11-02 |
| Measuring Wood: Board Feet (FEM-042) – University of Wisconsin–Madison Division of Extension | One board foot = 144 cubic inches (12 × 12 × 1 in). | 2026-11-02 |
| Regal Select Eggshell (N549) Technical Data Sheet – Benjamin Moore | Regal Select Eggshell interior wall paint: "Spread Rate Per Gallon 400 – 450 Sq. Ft." (preset uses 400). | 2026-11-02 |
| Waterborne Ceiling Paint Ultra Flat (N508) Technical Data Sheet – Benjamin Moore | Ceiling paint: "Spread Rate Per Gallon 400 – 450 Sq. Ft." (preset uses 400). | 2026-11-02 |
| SuperPaint Interior Latex Flat (A86 Series) Product Data Sheet – Sherwin-Williams | Interior wall paint: "Coverage: 350-400 sq. ft. per gallon" (preset uses 350). | 2026-11-02 |
| ProMar Ceiling Paint Flat (A27W05050) Product Data Sheet – Sherwin-Williams | Ceiling paint: "Coverage: 350-400 sq. ft. per gallon" (preset uses 350). | 2026-11-02 |
| Speedhide Interior Latex Flat (6-70 Series) Technical Data – PPG | Walls and ceilings: "Approximately 400 sq. ft. (37 sq. meters) per U.S. Gallon (3.78L) on smooth, nonporous surfaces." | 2026-11-02 |
| Valspar Signature Interior Flat (No. 773955 Series) Product Data Sheet – Valspar | Interior wall paint: "Practical Coverage: 400 sq. ft. per gallon (27-37 m2 per 3.78 L) depending on surface porosity." | 2026-11-02 |
| Harmony Product Data Sheet – Tikkurila | Walls and ceilings: "Theoretical coverage 8-12 m²/l." (preset uses 8). | 2026-11-02 |
| QUIKRETE Concrete Mix (No. 1101) Product Data Sheet – The QUIKRETE Companies | Yield per bag: 40 lb ≈ 0.30 ft³, 50 lb ≈ 0.375 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³, 90 lb ≈ 0.675 ft³. | 2026-11-02 |
| Technical Note 10 – Dimensioning and Estimating Brick Masonry – Brick Industry Association | Modular brick 3⅝ × 2¼ × 7⅝ in with ⅜ in joints, nominal 4 × 2⅔ × 8 in; 675 bricks per 100 sq ft (Table 4); waste "at least 5 percent". | 2026-11-02 |
| TEK 04-02A – Cost Estimating Guide for Concrete Masonry – Concrete Masonry & Hardscapes Association (formerly NCMA) | Nominal 8 × 16 in face = 8/9 sq ft per unit; 112.5 units per 100 sq ft before waste, 119 with a 5 % allowance. | 2026-11-02 |
| Sheetrock Brand Panels – Submittal Sheets – USG | Panels 4 ft wide (also 54 in) in lengths of 8 to 16 ft depending on product; USG environmental documentation assumes 10 % installation waste. | 2026-11-02 |
| How to Hang Wallpaper – Brewster Home Fashions | "Measure the height of your wall and add 4 inches … You will need an extra 2 inches at the top and bottom for trimming." | 2026-11-02 |
| Residential Asphalt Roofing Manual – Design and Application Methods (2014) – Asphalt Roofing Manufacturers Association (ARMA) | "1 square = 100 square feet"; "There are typically 3, 4 or 5 bundles per square." Check the wrapper of your product. | 2026-11-02 |
| TruDefinition Duration MAX Shingles – Product Data – Owens Corning | Coverage 98.4 sq ft per 4 bundles: bundles per square differ between products. | 2026-11-02 |
| Estimating How Much Roofing Is Required – IKO | Actual area = horizontal area × area/rake factor; 6/12 → 1.118. The factor equals √(1 + (rise/run)²) (Pythagoras). | 2026-11-02 |
| DIY Roofing: How to Measure a Roof – Owens Corning | "Plane Area x Correction Factor = Roof Area"; slope correction factor 6:12 → 1.118. | 2026-11-02 |
| 2026 Trex Decking Installation Guide (p. 24); Trex Enhance product page – Trex Company | Minimum width-to-width gap of 3/16 in (5 mm); Trex Enhance boards are 5.5 in wide. | 2026-11-02 |
| Aggregate unit weights – WisDOT research report 0092-22-05 – Wisconsin Department of Transportation | Compacted aggregate unit weights of about 1.5–2.2 tons per cubic yard depending on gradation; loose material weighs less. | 2026-11-02 |
| Mulch calculation (area × depth in inches ÷ 324 = cubic yards) – University of California Agriculture and Natural Resources | Cubic yards of mulch = square feet × depth in inches ÷ 324. | 2026-11-02 |
| Keracolor FF – Technical Data Sheet – MAPEI | "(A + B)/(A x B) x C x D x 1.5 = kg/m²" with tile length A, width B, thickness C and joint width D in mm; 1.5 is this product's factor (Keracolor GG uses 1.6). | 2026-11-02 |
| Grout Calculator / Project Estimator – LATICRETE | Adds 10 % to grout estimates for waste, spillage and clean-up. | 2026-11-02 |
| How Many Square Feet in a Pallet of Sod? – Sod Solutions | Slabs 16 × 24 in (≈ 2.67 sq ft), rolls of 5 or 10 sq ft; pallets commonly 400–700 sq ft depending on farm and piece type. | 2026-11-02 |
| Fiberglass Building Insulation Fact Sheet – Johns Manville | Coverage per bag differs by product, e.g. R-13 15 × 93 in batts: 106.56 sq ft per bag. | 2026-11-02 |
| How to Estimate Accessories for Your Siding Job – Ply Gem | For double 4 in siding, 12 pieces ≈ 1 square (100 sq ft); a square is 100 sq ft of wall. | 2026-11-02 |
| Solidify II 12′ and Sophistication 15′ broadloom carpet – product specifications – Shaw Floors | US broadloom carpet is sold in fixed roll widths, e.g. 12 ft and 15 ft products; the width is listed per product. | 2026-11-02 |
| Broadloom vs. carpet tiles: which should you choose? – ITC (carpet manufacturer) | Broadloom carpet comes in rolls 4–5 m wide. | 2026-11-02 |
| RollOver 27 collection – roll sizes – Erismann & Cie. | European wallpaper rolls of 0.53 × 10.05 m (also 0.75–1.06 × 25 m for wide rolls). | 2026-11-02 |
| How to estimate wallpaper – Sherwin-Williams | US wallpaper is priced in single rolls (about 25 usable sq ft) but sold in double or triple rolls; roll sizes vary by product. | 2026-11-02 |
| Thistle MultiFinish – Product Data Sheet (BG-PDS-110-04) – British Gypsum | Example plaster coverage: 10 m² per bag at 2 mm thickness (coverage is product-specific). | 2026-11-02 |
| Plywood – APA – The Engineered Wood Association | Traditional nominal panel size 48 × 96 in (4 × 8 ft); some panels are made slightly smaller for spacing. | 2026-11-02 |
| Metsä Wood Birch – product datasheet – Metsä Wood | Standard panel formats 1200/1220/1250 mm wide, 2400–3660 mm long, to EN 315. | 2026-11-02 |
| NDS Supplement (2018), Table 1A – Nominal and Minimum Dressed Sizes of Sawn Lumber – American Wood Council | Dry dressed sizes: nominal 1 in → 3/4 in, 2 in → 1-1/2 in; widths 4 → 3-1/2 in, 6 → 5-1/2 in. | 2026-11-02 |
| 100mm Dense Concrete Block 22.5N/mm² – Stowell Concrete | UK concrete blocks 440 × 215 mm face, made to BS EN 771-3. | 2026-11-02 |
| Scotts Nature Scapes Mulch 2 cu ft; Scotts Premium Topsoil – ScottsMiracle-Gro | Example bag sizes: mulch 2 cu ft (https://scottsmiraclegro.com/en-us/8840244.html), topsoil 0.75 cu ft. | 2026-11-02 |