Changelog and corrections

We publish the errors we find. If a calculator here ever gave a wrong answer, this page says what it was and when it was fixed. How we check our work is described on the methodology page, and you can report a problem at any time.

1. October 11, 2026

   Source audit: tables and calculator constants re-checked against codes and manufacturer data. We re-checked every reference table and the key constants behind each calculator. Corrections: tile calculator grout now uses 0.064 lb per cubic inch of joint, matching the Custom Building Products coverage card (it was 0.21, which overstated grout about 3.3 times), and 1/2 in notch thinset covers 40 ft² per bag (was 45). Wallpaper: a 27 in × 27 ft roll is a US double roll, not a single, so the calculator now counts rolls as sold and shows the US single-roll count for pricing (it previously halved the order for US paper). Insulation: blown-in bag coverage at R-38 is now 51 ft² for fiberglass and 24 ft² for cellulose (one 40 ft² figure was used for both, under-buying cellulose by about 40%). IECC 2021 wood-frame walls in zones 4–8 now read "30 or 20+5ci or 13+10ci or 0+20ci"; climate zones corrected for 14 states, including New Hampshire, West Virginia and Wisconsin, whose main zone is 5, 4 and 5; R-49 batts are about 13-1/2 to 14 in thick. Water heater sizing uses the current DOE worksheet (hand dishwashing or food prep 3 gal, dishwasher 7, clothes washer 25), and an 80 gal electric tank is no longer suggested for first hour ratings above 80. The BTU calculator never goes below the ENERGY STAR room air conditioner chart, so small rooms get a larger, more realistic unit. NEC 2023: box fill no longer lists a separate allowance for isolated-ground conductors (removed in 2020) and adds terminal blocks (314.16(B)(6)), shows the printed Table 314.16(A) maximum of five 8 AWG conductors in a 4 × 1-1/4 in round box (the volume arithmetic gives four), and no longer counts grounding conductors that enter from a domed luminaire canopy; voltage-drop notes are cited at 210.19 and 215.2(A)(2); the electrical load calculator adds the 40% factor for four or more separately controlled electric heaters (220.82(C)). Also updated: drywall weights (1/4 in 1.2 and 3/8 in 1.4 lb/ft²) and tape (370 ft per 1,000 ft²), primer coverage (300 ft²/gal), roofing nail weight (about 1.45 lb per square), roof H-clips (one per rafter bay, 4 per 4 × 8 sheet), compost and hardwood mulch weights, and typical first hour ratings. Each page lists the source it was checked against.

2. October 11, 2026

   Site rebuilt from scratch; all calculators re-derived from written specs. The previous prototype was retired. Every calculator is now written from a specification with independently worked examples, audited against it and covered by automated tests before it goes live. Calculators are published trade by trade as they pass those checks.