How we calculate

Our goal is that you can trust a result enough to buy materials from it. Here's how we get there.

Formulas

Every calculator uses standard geometry (areas, volumes and the Pythagorean theorem) plus published product data. The calculations are written once and shared by the calculator, the worked examples on each page and our automated tests, so the numbers in the text always match what the tool shows.

Data sources

• Concrete: bag yields from manufacturer labels: 0.30 / 0.45 / 0.60 cu ft for 40 / 60 / 80 lb standard mix, and 0.375 cu ft for 50 lb fast-setting mix.
• Stairs: International Residential Code (IRC) R311.7 limits: 7¾" maximum riser, 10" minimum tread, ⅜" maximum variation, handrail at four or more risers.
• Paint: spread rates from manufacturer data sheets (350–400 sq ft per gallon on smooth surfaces).
• Tile grout: the industry formula (A + B) ÷ (A × B) × thickness × joint × 1.6 kg/m², used on grout manufacturers' data sheets.
• Drywall finishing: standard estimating rates per 1,000 sq ft of board (≈138 lb ready-mixed compound, ≈370 ft tape).
• Retaining walls: block sizes, weights and cap sizes from Allan Block, Anchor Wall Systems and Belgard (Diamond Pro), Versa-Lok (General Shale bulletin) and Pavestone product data. Base trench, burial depth, drainage zone, drain pipe and height limits from the Allan Block residential and commercial installation guides, Anchor's Diamond Pro and multi-piece installation guides, and ICC-ES report ESR-1959. Gravel tons use 1.4 tons per cubic yard, as in our gravel calculator.
• Sod and grass seed: sod piece and pallet sizes from Sod Solutions and The Grass Outlet; soil preparation and watering from Clemson, Georgia (B1533), Missouri and Nebraska extension. Seeding rates per 1,000 sq ft from University of Georgia B1533, University of Tennessee W160-B (tall fescue, ryegrass), University of Kentucky AGR-52 and University of Nebraska G1946 (buffalograss); overseeding rates from Suffolk County (NY) Cornell Cooperative Extension and Georgia; warm-season overseeding uses NC State's rule of 25 to 50% below the new-lawn rate.
• Concrete block: 1.125 blocks per square foot for 8 × 8 × 16 in units, actual dimensions and grout volumes from the Concrete Masonry & Hardscapes Association (CMHA, formerly NCMA) TEK 04-02A and TEK 03-11. Mortar yields per 80 lb bag from the QUIKRETE Mortar Mix and Sakrete Type S data sheets and the CMHA estimate for preblended mortar.
• Gutters: slope, hanger spacing and section lengths from the Amerimax metal and vinyl gutter installation guides and Spectra's installation guide. The minimum of one downspout per 40 ft of gutter comes from Amerimax's vinyl guides; we apply the same rule of thumb to metal gutters. Three elbows per downspout is the usual layout (two at the eave, one at the bottom).
• Frost line depth: each state's adopted residential code (Table R301.2 or the state rule that replaces it), read from official state sites, state administrative code, or UpCodes copies of the adopted code, with each link on the page. Where a state leaves frost depth to local governments, we list only city values read in official city code copies or city documents. General rules quote the 2021 International Residential Code, Sections R403.1.4, R403.1.4.1 and Table R301.2.
• Frost dates: NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020 (annual/seasonal normals, version 1.0.1). We use NOAA's published 32°F and 28°F spring and fall probability dates for all 7,305 stations that have them, unchanged; frost-free days are the gap between the two median 32°F dates. Each ZIP code is matched to the nearest station by great-circle distance from the ZIP's Census internal point (2020 Census Gazetteer); cities use the ZIP code nearest their Census center point. Where NOAA flags frost as too rare (under 10% of years) or possible every day of the year, we show that instead of a date. Where frost happens in only some years, NOAA's dates describe those years only, and we label them that way.
• Planting calendar: weeks to grow seedlings indoors and when to set them out come from UNH Cooperative Extension, Growing Vegetables: When to Plant your Vegetable Garden (2016). Peas, spinach, beans, sweet corn, carrots and beets follow Colorado State University Extension, CMG GardenNotes #720: Vegetable Planting Guide (2018, reviewed 2024). Dates are counted from your average last frost.
• Fall planting calendar: latest planting dates, in weeks before the first fall frost, and days to harvest from Montana State University Extension MontGuide MT199308AG, Can I Grow That Here? Vegetable Seed and Transplant Schedule (reviewed 2013). Frost tolerance from Purdue Extension HO-66, The Fall Vegetable Garden. The counting-back method (days to harvest plus about two weeks for slower fall growth) follows Virginia Cooperative Extension publication 426-334 and Nebraska NebGuide G1343.
• Planting zones: USDA Plant Hardiness Zone Map, 2023, Agricultural Research Service, U.S. Department of Agriculture (planthardiness.ars.usda.gov). The ZIP code table comes from the PRISM Group, Oregon State University (prism.oregonstate.edu, accessed 23 Oct 2026), and matches USDA's own ZIP lookup for all 40,502 ZIP codes. The "average for this ZIP" temperature is from USDA's ZIP table, shown only when it falls inside the ZIP's zone. State and city lists are our own summaries, not the official USDA map. Place names shown for ZIP codes, and states for ZIP codes without a Census area, come from GeoNames (CC BY 4.0).
• Climate zones: every county's zone comes from Pacific Northwest National Laboratory's Guide to Determining Climate Zone by County: Building America and IECC 2021 Updates (2022, OSTI 1893981). Nine county-equivalents missing from that guide (including Ada County, ID and Sussex County, DE) come from the 2021 IECC Table R301.1 itself, checked against state adoptions of the code. The ZIP code and city search uses US Census Bureau files that place each ZIP code (ZCTA) and city in its county.
• Insulation requirements: 2021 IECC Table R402.1.3 (the corrected 2nd/3rd printing, which adds the R-30 wall option) and 2018 IECC Table R402.1.2. Every cell was checked against the International Code Council's text and several states' unamended adoptions. Attic recommendations for existing homes come from ENERGY STAR. R-value per inch for blown insulation uses US Department of Energy ranges (fiberglass about R-2.2 to 2.7, cellulose about R-2.9 to 3.8); we use R-2.5 and R-3.2 so depth estimates aren't understated. Which code edition each state enforces comes from the DOE Building Energy Codes Program. Batt thicknesses come from Owens Corning and Johns Manville product data sheets.
• Paver patios: base, bedding sand and edge rules from the Concrete Masonry & Hardscapes Association (CMHA, formerly ICPI) technical specs PAV-TEC-002 and PAV-TEC-003; paver, sand and edging coverage from manufacturer data (Unilock, Pavestone, Belgard, Nicolock, Rochester, Sakrete, Gator, Dimex, Snap Edge).
• Insulation quantities: batt coverage from Owens Corning, Johns Manville and ROCKWOOL data sheets; blown-in coverage from the Owens Corning AttiCat and Greenfiber SANCTUARY bag charts.
• Energy codes by state: US Department of Energy Building Energy Codes Program state status data.
• Bulk materials: typical loose densities (e.g. gravel ≈ 1.4 tons per cubic yard). Moisture and stone type change these by 10–20%.
• Sand: 50 lb bags at about 0.5 cu ft, as listed for Quikrete play sand and all-purpose sand by retailers (Lowe's, Tractor Supply), which gives 100 lb per cu ft and 1.35 tons per cubic yard; dry and wet sand densities (97 and 119 lb per cu ft) from Engineering ToolBox; product uses from the Quikrete Sand & Gravel data sheet; 1-inch bedding sand from CMHA PAV-TEC-002.
• Cubic yards: exact conversions (27 cu ft per cubic yard, 35.3147 cu ft per cubic meter). Weights use the bulk densities above.

Rounding and waste

We always round up to whole bags, boxes and boards, because you can't buy part of one. Waste allowances follow common trade practice and can be adjusted in most calculators.

Limits

Results are planning estimates. Structural elements such as footings, stairs, decks and roofs must meet your local building code, and your building department has the final say. When in doubt, consult a licensed professional.

Last updated October 21, 2026.