How BuildLumi Calculators Work
This page explains where the formulas come from, how units are converted, when numbers are rounded, and what the calculators deliberately do not do.
Where formulas and data come from
BuildLumi uses the most authoritative source available for each number, in this order:
- Government and standards bodies. Unit definitions come from NIST Handbook 44, Appendix C.
- Manufacturer data sheets and product guides. Bag yields, shingle coverage and joint-material rates come from the sheets that ship with the product.
- University extension publications. Mulch and soil volume guidance comes from extension services.
- Plain geometry. Areas, volumes and pitch factors are mathematics, not data.
- Industry conventions. Waste allowances and some material densities have no official standard. They are labeled as conventions and are always editable.
| Calculator | What the source supports | Source |
|---|---|---|
| All calculators | Exact unit conversions | NIST Handbook 44, Appendix C |
| Concrete | Bag yield per weight of bag; 150 lb/ft³ default weight of plain concrete | QUIKRETE Concrete Mix #1101 data sheet; WSDOT Bridge Design Memo 05-2010 |
| Mulch | Volume formula; 3 in depth; coverage per yard | Texas A&M AgriLife Extension; UF/IFAS Gardening Solutions |
| Drywall | Tape, joint compound and screws per 1,000 ft² | CGC/USG Construction Handbook, ch. 3; Georgia-Pacific ToughRock Wallworks Guide (manufacturer guide, used as a cross-check) |
| Roofing | Bundles per square, coverage, ridge, starter and underlayment | Owens Corning Roofing Collection Product Guide |
| Fence | Posts, rails and pickets per run; post hole size and depth | Outdoor Essentials fence calculator guide; QUIKRETE Fast-Setting Concrete Mix data sheet |
| Deck | Board spacing and joist spacing options (layout only) | Trex deck planning guide |
| Square footage, cubic yard, roof pitch | Geometry and exact conversions | Mathematics and NIST Handbook 44 |
| Gravel, topsoil | Geometry is exact. Material weights have no official standard (medium to low confidence). | Supplier-style density tables, shown as editable estimates: Crushed Stone Calculator (secondary source) and Inch Calculator (secondary source). Soil volume and bag sizes: Texas A&M AgriLife Extension. |
| Tile | Tile count is geometry. Thinset coverage per 50 lb bag by trowel size comes from one manufacturer’s chart (higher confidence for that product; other brands differ). Grout is an estimate calibrated to one manufacturer’s coverage chart (medium confidence). The 10%, 15% and 20% waste presets are retailer guidance and convention (medium to low confidence). | CUSTOM Building Products VersaBond Professional Tile Mortar (TDS-119); CUSTOM Polyblend Plus Sanded Grout (TDS-129); Lumber Liquidators flooring estimate guide (retailer guidance) |
| Siding | Geometry and the 100 ft² siding square are higher confidence (manufacturer guide). The 200 ft² vinyl carton and the fiber-cement plank (8.25 in wide, 7 in exposure, 12 ft long) are medium confidence: they come from retailer and editorial pages, because no manufacturer carton or exposure table could be opened. Both are editable, so read your carton or the manufacturer’s guide. The 10% waste is a convention. | Ply Gem siding estimating guide; Hunker (secondary source); Inch Calculator (secondary source); retailer specification for HardiePlank HZ5 |
| Asphalt | A typical spread rate of 110 lb per square yard per inch of compacted thickness, with a published range of 108 to 120. Higher confidence as a typical figure; your mix design sets the real one. | Asphalt Magazine (Asphalt Institute); Pennsylvania Asphalt Pavement Association; Tennessee DOT |
| Cubic yards to tons | Each preset carries a confidence label. Plain concrete at 150 lb/ft³ (2.025 tons per yd³) and compacted asphalt at about 1.98 tons per yd³ are higher confidence. Crushed stone, crusher run, pea gravel and river rock are medium confidence (supplier-style ranges). Sand, soil and fill dirt are low confidence (convention). All are editable. | WSDOT Bridge Design Memo 05-2010; Asphalt Magazine; Crushed Stone Calculator (secondary source); Inch Calculator (secondary source) |
| Board foot | The board foot definition (1 in × 12 in × 12 in, or 144 in³) and the formula are exact. Actual sizes come from the American Softwood Lumber Standard (PS 20) as compiled by ArchToolbox, a secondary source that agrees with SPIB’s text (medium to higher confidence). Whether a seller prices softwood on nominal or actual size is a trade convention that sources disagree on (medium confidence), so both are shown and nominal is the default. | SPIB, Nominal vs. Actual Lumber Sizes; ArchToolbox, Lumber Dimensions (secondary source); Penn State Extension, Valuing Standing Timber; Ohio State University Extension, Measuring Standing Trees |
| Brick | Bricks per 100 ft² (675 modular, 655 standard-size), the 5% minimum brick waste, the 15 to 25% mortar waste and the bond example come from BIA Technical Note 10 (higher confidence). The modular mortar quantity of 5.5 ft³ per 100 ft² was read from a table whose text was hard to extract and cross-checked by geometry (medium to higher confidence). The 37 bricks per 80 lb bag is one product’s label. | Brick Industry Association, Technical Notes on Brick Construction 10; QUIKRETE Mortar Mix No. 1102 data sheet |
| Concrete block | Blocks per square foot (1.125 for an 8 × 8 × 16 in block), the 5% allowance, the NCMA mortar rate of 16 blocks per bag and the core-fill grout table come from NCMA TEK 04-02A (higher confidence). The default of 13 blocks per bag is one product’s label and is the more conservative figure. Mortar and grout are given for the standard block only. | NCMA TEK 04-02A, Estimating Concrete Masonry Materials (hosted by CMHA); QUIKRETE Mortar Mix No. 1102 data sheet |
| Paint | Spread rates are manufacturers’ own figures for named products: 350 to 400 sq ft per gallon for Sherwin-Williams SuperPaint and 400 to 450 for Benjamin Moore Regal Select (higher confidence), and 400 for the Sherwin-Williams primer. Behr’s 250 to 400 comes from a search summary of its page, so it is lower confidence and labeled that way. The calculator uses the low end of each range. The 20 sq ft door and 12 sq ft window are nominal conventions you can edit. | Sherwin-Williams SuperPaint data sheet 101.02A; Sherwin-Williams primer data sheet 108.38; Benjamin Moore Regal Select; Behr Premium Plus (lower confidence); NIST Handbook 44 for the gallon and quart |
| Linear feet | The conversion between square feet and linear feet is arithmetic and exact. Actual widths are PS 20 dry, surfaced sizes as compiled by ArchToolbox, a secondary source (medium to higher confidence). The 10% default allowance and the 8, 10, 12 and 16 ft stock lengths are conventions: retailer guidance puts straight-lay flooring waste at 7% to 10%, and 10% is a common recommendation for baseboard. | ArchToolbox, Lumber Dimensions (secondary source); Lumber Liquidators flooring estimate guide (retailer guidance); This Old House, How to Install Baseboards; Miniwebtool Baseboard Trim Calculator (secondary source) |
Confidence labels. Higher means a standard, a government document or a manufacturer data sheet. Medium means a single manufacturer chart for one product, a retailer page or an editorial page. Low means a convention with no primary source. Pages that use a secondary source, such as a competing calculator that cites no primary document, say so next to the link. Where a primary source exists for the same figure, BuildLumi uses that instead.
Product names appear only to identify the documents used. BuildLumi is not affiliated with those companies, and a yield on a data sheet applies to that product only. Check the label on your own bag.
Units and conversions
Calculators work internally in US customary units (feet, square feet, cubic feet, pounds) and convert to metric only for display, using the exact factors below.
| Quantity | Exact value |
|---|---|
| 1 inch | 2.54 cm (25.4 mm) |
| 1 foot | 12 in = 0.3048 m |
| 1 square foot | 144 in² = 0.09290304 m² |
| 1 acre | 43,560 ft² = 4,046.8564224 m² |
| 1 cubic foot | 1,728 in³ = 0.028316846592 m³ = 28.316846592 L |
| 1 cubic yard | 27 ft³ = 46,656 in³ = 0.764554857984 m³ |
| 1 US gallon | 231 in³ = 3.785411784 L |
| 1 pound | 0.45359237 kg |
| 1 ton (short ton) | 2,000 lb = 907.18474 kg |
In US construction, “ton” means the short ton of 2,000 lb. A metric tonne is 1,000 kg, and a long ton is 2,240 lb. BuildLumi labels tons and tonnes so they cannot be confused. Gallons are US gallons; an imperial gallon is about 20% larger.
Rounding
- Full precision is kept through every step. Only the number you see is rounded for display.
- Things you buy in whole units, such as bags, sheets, bundles, rolls, gallons and pieces of trim, always round up, and the allowance is added before rounding.
- Tiny floating-point noise is removed before rounding up, so a result of 67.00000000001 bags is not shown as 68.
Example you can reproduce
A 10 ft × 12 ft slab, 4 in thick, with a 10% allowance, poured with 80 lb bags that yield 0.60 ft³ each:
volume = 10 × 12 × (4 ÷ 12) = 40 ft³ = 1.4815 yd³
with 10% = 40 × 1.10 = 44 ft³ = 1.6296 yd³
bags = 44 ÷ 0.60 = 73.33, rounded up to 74Allowances and waste
Waste, compaction and overage percentages are rules of thumb, not standards. Each calculator shows its default next to the field. Treat the numbers below as starting points and change them for your project.
| Calculator | Default | Basis |
|---|---|---|
| Concrete | 10% (12% to 15% for complex pours) | Convention |
| Cubic yard | 0% (exact volume) | You add an allowance if you need one |
| Gravel | 15% compaction, 0% spillage | Convention. Shown separately so the two do not stack unnoticed. |
| Mulch | 5% | Convention |
| Drywall | 10% | Convention |
| Roofing | 10% simple gable, 15% hip or complex; 5% for ridge and starter | Convention |
| Fence pickets | 10% extra | Recommended in the Outdoor Essentials guide |
| Deck boards | 10% | Convention |
| Topsoil | 10% for settling | Convention |
| Asphalt | 5% | Convention. Covers edges, thickness variation and truck-load rounding. |
| Tile | 10% straight grid, 15% diagonal, 20% herringbone or mosaic; 10% extra on grout | Convention and retailer guidance. There is no tile standard for it. |
| Siding | 10% | Convention. Gables and many openings push it higher. |
| Cubic yards to tons | None | A conversion, not an order quantity |
| Board foot | None | A volume, not an order quantity. Add your own allowance for cuts and defects. |
| Brick | 5% bricks; 20% mortar | BIA: at least 5% added to the net brick count as a general rule, and 15 to 25% to the net mortar. The 20% is the middle of that range. |
| Concrete block | 5% | NCMA includes about 5% for waste and breakage in its block counts. Its grout table includes 3%. |
| Paint | No waste field. Two coats, and the low end of each stated spread rate | Convention. The estimate errs high, and gallons round up. Primer is one coat. |
| Linear feet | 10% (0% to 50%) | Convention. Retailer guidance puts straight-lay flooring waste at 7% to 10%, and 10% is common for baseboard. |
Densities, yields and coverage
Weights and coverage are estimates. Gravel, sand and soil vary with rock type, gradation and moisture, and no government or manufacturer standard exists for them, so BuildLumi uses typical supplier-style values and lets you type your supplier’s figure. Bag yields, shingle coverage and joint-material rates are product-specific. The calculators use the data-sheet value where one was verified and ask you for the number on your package where one was not.
The newer calculators follow the same rule. Thinset coverage and grout use depend on the product, so the tile calculator labels them as one manufacturer’s chart and an estimate. A vinyl siding carton of 200 ft² and a fiber-cement plank exposure of 7 in are medium confidence, from retailer and editorial pages, and both are inputs you can change. The cubic-yards-to-tons converter shows a confidence label on every material: higher for concrete and asphalt, medium for crushed stone and related aggregates, low for sand, soil and fill dirt. The brick and block calculators give mortar bags from one product’s label and show the BIA volume or the generic NCMA rate separately, because the figures differ. The board foot calculator shows nominal and actual size side by side because sources disagree on which one prices lumber. The paint calculator uses the low end of each manufacturer’s stated spread rate and lets you type the coverage from your own can, because coverage depends on the surface and the product. The linear feet calculator is arithmetic, and the only data in it are the actual widths you enter.
How the calculators are tested
Each calculator has automated tests that compare its output with examples computed independently of the calculator code. Tests also cover the edge cases: empty and zero inputs, negative or non-numeric values, unit switches, and rounding right at a whole number.
What the calculators do not do
BuildLumi estimates quantities only. It does not design structures or systems. In particular it does not size footings or slabs for loads, soil or frost depth, choose rebar, calculate beam, joist or rafter spans, design deck footings, ledgers or railings, design masonry walls, lintels or ties, or cover electrical, plumbing or fire-rated assemblies. See the calculation disclaimer.
Updates and corrections
Sources were last verified on . When a manufacturer changes a product or a reader reports a problem, the number is checked against the original document before it is changed. To report an error, write to [email protected] and include the calculator, your inputs and the result you expected.