DIY Project CostUS project-cost lab

Method

Formulas, waste, and why we refuse invented prices

All math lives in src/data/ and is covered by npm test. Calculators hydrate in the browser. We never see your dimensions. Index date: 2026-08-27.

Paint

Net sq ft = 2 × (L + W) × H − 20×doors − 15×windows [+ L×W if ceiling]
Gallons   = (net × coats × (1 + waste%)) ÷ coverage
Buy       = ceil(gallons)

Default coverage is 350 sq ft per gallon per coat — PaintCare’s fallback when the can is silent, sitting inside the Sherwin-Williams Emerald/SuperPaint and Benjamin Moore Ultra Spec 350–400 band. PPG SPEEDHIDE is specified around 400, not 350; do not collapse it. Behr publishes 250–400 for one coat; the system recommendation is still two coats. 350 is not an EPA figure.

Door 20 sq ft and window 15 sq ft are Behr how-to averages. 10% waste is a calculator convention. Sherwin-Williams’ paint-calculator FAQ only says you may want a little extra on hand.

Concrete

cu ft = L × W × (depth_in / 12)
80 lb bags = ceil(cu ft / 0.60)     45 bags / yd³
60 lb bags = ceil(cu ft / 0.45)     60 bags / yd³

Yields: Quikrete 1101 data sheet, revised October 2022, and Sakrete High-Strength TDS. The Quikrete calculator method has no waste percent — round up to the next bag. 10% here is an optional toggle.

There is no official 2026 NRMCA dollars-per-yard in this index. Secondary blogs quote about $125–$195; labelled unofficial. FRED WPU1333 July 2026 = 394.468 is a producer price index, not a retail cubic-yard price.

Deck

Actual 5/4×6 = 1.0 × 5.5 in
LF / sq ft (no gap) = 12 / 5.5 = 2.182
Boards = ceil(width_in / 5.5) × ceil(length / board_length)
Buy    = ceil(boards × (1 + waste%))

Trex’s published extra is 5%. Some retailer guides use 10% straight / 15% diagonal. PT deck board and 2×8 joist prices are null (zip-localized). We will not invent them.

Drywall

4×8 sheet = 32 sq ft     (USG J371: 10 sheets = 320)
Net = walls + ceiling − doors − windows
Sheets = ceil(net × (1 + waste%) / 32)

USG has no waste percent. Subtract doors and windows; do not subtract electrical boxes. Opening sizes 20 / 15 sq ft are carried over from Behr so the 12×12 worked rooms match the paint tool.

Not an engineer

These pages estimate DIY material quantities. They do not size beams, mix designs, hanging patterns, or fire assemblies. If a number is missing from a manufacturer PDF or a dated shelf ticket, the UI shows the formula and an empty price field.

Manufacturer PDFs and primary sources

  1. PaintCare. Buy the Right Amount of Paint to Reduce Waste. Accessed 2026-08-27. https://www.paintcare.org/buy-right/ — If the can does not list coverage, PaintCare says use 350 sq ft per gallon as a rough estimate. Coverage on cans is typically 300–400 sq ft/gal. This is the site default fallback — not an EPA figure.
  2. Sherwin-Williams. Emerald Interior Acrylic Latex product data sheet. Accessed 2026-08-27. https://www.sherwin-williams.com/document/PDS/en/035777628072/ — Coverage: 350–400 sq. ft. per gallon @ 4 mils wet, 1.5 mils dry. One coat at that film thickness.
  3. Sherwin-Williams. SuperPaint Interior Latex Satin product data sheet. Accessed 2026-08-27. https://www.sherwin-williams.com/document/PDS/en/035777315682/ — Coverage: 350–400 sq. ft. per gallon @ 4 mils wet, 1.5 mils dry.
  4. Sherwin-Williams. How to Use Paint Calculator | FAQs. Accessed 2026-08-27. https://www.sherwin-williams.com/en-us/color/color-tools/paint-calculator — A gallon typically covers about 350–400 sq ft. SW says depending on texture and desired coverage, keeping a little more than a gallon on hand may be best. That is not a numeric waste percentage.
  5. Benjamin Moore. Ultra Spec interior latex coverage (350–400 sq ft/gal). Accessed 2026-08-27. https://www.benjaminmoore.com/en-us/specifications-and-data/ultra-spec — Ultra Spec architectural coatings list 350–400 sq ft per gallon on product data. Same one-coat spread-rate band as SW Emerald/SuperPaint.
  6. PPG. SPEEDHIDE interior latex spreading rate. Accessed 2026-08-27. https://www.ppgpaints.com/products/speedhide-interior-latex — SPEEDHIDE interior latex is specified around 400 sq ft/gal, not 350. Do not mix this rate into the PaintCare 350 fallback.
  7. Behr. How Much Interior Paint to Buy for your Home. Accessed 2026-08-27. https://www.behr.com/consumer/how-to/interior/determine-how-much-interior-paint-to-buy — Average window about 15 sq ft; average door about 20 sq ft. How-to copy discusses one coat coverage; Behr system recommendation is two coats for most color changes and new work.
  8. Behr. How Much Paint Do I Need? (exterior). Accessed 2026-08-27. https://www.behr.com/how-to/exterior/how-much-exterior-paint-to-buy — One gallon of any BEHR paint covers 250–400 sq ft in one coat. Same 15 / 20 sq ft opening allowances. Factor a second coat if necessary.
  9. The Quikrete Companies. QUIKRETE Concrete Mix product data (No. 1101). Accessed 2026-08-27. https://www.quikrete.com/pdfs/data_sheet-concrete%20mix%201101.pdf — Revised October 2022. An 80 lb bag yields approximately 0.60 ft³; a 60 lb bag yields approximately 0.45 ft³. The published calculator method is divide volume by yield and round up to the next bag — no waste percentage.
  10. Sakrete. Sakrete High-Strength Concrete Mix technical data sheet. Accessed 2026-08-27. https://www.sakrete.com/wp-content/uploads/2024/01/Sakrete-High-Strength-Concrete-Mix-TDS.pdf — 80 lb bag yield 0.6 ft³; 60 lb bag 0.45 ft³. Confirms the same 80 lb / 0.60 ft³ industry bag used in the Quikrete 1101 sheet.
  11. USG. Sheetrock Brand gypsum panels submittal J371. Accessed 2026-08-27. https://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/sheetrock-brand-gypsum-panels-submittal-J371.pdf — 4 × 8 ft panels: 10 sheets = 320 sq ft, so one sheet is 32 sq ft. USG installation math divides area by panel square footage. No manufacturer waste percentage. Subtract doors and windows; do not subtract electrical boxes.
  12. USG / Home Depot PDF. USG Sheetrock Brand gypsum panels installation guide. Accessed 2026-08-27. https://images.thdstatic.com/catalog/pdfImages/9f/9fe15ca9-324e-4407-b4ef-9bdbc1946a6c.pdf — 4 ft × 8 ft = 32 sq ft; 4 × 10 = 40; 4 × 12 = 48. Divide measured area by panel area. No waste factor listed.
  13. USG. Type W screws for 1/2 in gypsum panels. Accessed 2026-08-27. https://www.usg.com/content/usgcom/en/products/walls/drywall.html — USG specifies 1-1/4 in Type W screws for 1/2 in single-layer panels — not 3 in coarse-thread drywall screws. The Grip-Rite 3 in 1 lb box in the price index is a shelf SKU, not the specified fastener.
  14. Trex. How Many Deck Boards Do I Need?. Accessed 2026-08-27. https://www.trex.com/deck-ideas/how-many-deck-boards-do-i-need-/ — Actual 5.5 in face width for typical 5/4×6 / 1×6 boards. General rule: 5% extra for waste. Trex’s own 12 ft width example: 144 / 5.5 = 26.18 → 27 boards; 5% extra rounds to 29.
  15. The Home Depot. WeatherShield 5/4 in × 6 in pressure-treated decking — actual size. Accessed 2026-08-27. https://www.homedepot.com/p/WeatherShield-5-4-in-x-6-in-x-16-ft-Standard-Ground-Contact-Pressure-Treated-Southern-Yellow-Pine-Decking-Board-253919/206968440 — Actual product thickness 1 in; actual product width 5.5 in. Matches Trex 5.5 in face width. No national online price (zip-localized).
  16. Composite Deck Direct / retailer how-tos. Composite decking measuring guides (10% straight / 15% diagonal). Accessed 2026-08-27. https://compositedeckdirect.com/blogs/composite/composite-decking-measuring-guide — Some retailer guides use 10% waste for straight layouts and 15% for diagonal. Distinct from Trex’s 5% rule of thumb. Offered as optional toggles, not as a manufacturer TDS.
  17. Custom Building Products. Custom VersaBond TDS-119 coverage. Accessed 2026-08-27. https://www.custombuildingproducts.com/products/setting-materials/polymer-modified-mortars/versabond — 50 lb bag covers about 85–95 sq ft at a 1/4 in square notch. Index lists the 50 lb bag retail price; this site does not include a tile calculator in v1.
  18. FRED, Federal Reserve Bank of St. Louis. Producer Price Index by Commodity: Nonmetallic Mineral Products: Concrete Ingredients and Related Products (WPU1333). Accessed 2026-08-27. https://fred.stlouisfed.org/series/WPU1333 — July 2026 value 394.468. This is an index number, not dollars per cubic yard. There is no official 2026 NRMCA national $/yd³ in this basket.
  19. Secondary cost blogs (not NRMCA). Unofficial 2026 ready-mix blog bands. Accessed 2026-08-27. https://latestcost.com/price-ready-mix-concrete-across-u-s-markets/ — Secondary blogs circulating in 2026 quote roughly $125–$195 per cubic yard for standard residential mix. Not an official NRMCA figure. Shown only as a labelled unofficial planning band; the field stays user-editable.

These calculators estimate material takeoff for planning. They are not engineering, structural, or code advice. Confirm spans, mixes, fastener schedules, and permits with a licensed professional and the manufacturer’s current data sheet. Local shelf prices and tax are not this index.