Our Commitment to Accuracy

The short version

• ✓Accuracy: formulas come from published codes, standards and manufacturer instructions, and are re-checked against the source text.
• ✓Transparency: each page shows its formulas, constants and sources, and code results name the exact table and edition.
• ✓Accountability: when we find an error, we fix it and log it publicly below.

Results are planning estimates. Real quantities depend on site conditions, the exact product and the installer, and your local building department has the final word on code. Check quantities with your supplier before you order.

How a calculation is built

How we check it

Prices

Calculators show materials only: quantities, not prices. Prices change too fast to keep accurate for every product in every market. Project costs live on separate cost guides, which show wide low-to-high ranges built from published sources, with the date they were last reviewed shown on the page.

Corrections log

Fixes that changed a number you might have relied on, newest first.

1. November 12, 2026

   #57 stone weight, now one figure site-wide

   • Five calculators weighed the same #57 stone four different ways: Gravel and Stone at 2,470 lb per cubic yard, Landscape Material at 2,700, Retaining Wall and Paver at 1.40 tons, and French Drain at 105 lb per cubic foot (2,835). All were inside the published range, but the same stone came out up to 15% different by calculator.
   • All five now use 2,700 lb per cubic yard (1.35 tons), toward the upper end of the published 2,410–2,835 range so orders don’t come up short. Gravel and Stone estimates for #57 go up about 9%; French Drain, Retaining Wall and Paver go down 3–5%.
2. November 12, 2026

   Drywall screw spacing (Drywall Screw and Drywall calculators)

   • The screw guide, FAQ, methodology and diagram gave screw spacing as 8″ at panel edges and 12″ in the field. GA-216 (§5.4) actually spaces screws evenly along every stud or joist: up to 16″ apart on walls and 12″ on ceilings (12″ on both with 24″ framing). The 8″ and 7″ figures are the nail rule (§5.2).
   • The diagram also showed ceiling joists at 12″ o.c.; ceilings are typically framed at 16″ or 24″, and 12″ is the screw spacing. Redrawn.
   • The guide’s per-sheet count said a vertical 4×8 sheet hits 5 studs and its table added up to 54+ screws while the total said 28–32. A 4×8 sheet on 16″ studs hits 4 studs × 7 screws = 28.
   • Drywall Calculator: ceiling screws were figured at 1.5 per sq ft while the results panel said 1.25. Now 1.25 (40 per sheet), matching the Drywall Screw calculator.
   • Metal-framing screw penetration is ⅜″ minimum per GA-216, not ¼″.
3. November 12, 2026

   Quick-answer review: drywall, backsplash, siding, framing, French drain

   • Drywall Calculator: tape was figured from every sheet’s full perimeter, counting each joint twice (about 990 ft for 1,000 sq ft). It now uses USG’s ≈370 ft per 1,000 sq ft, the same figure as the Drywall Mud calculator.
   • Drywall guide: tape said 5,000 ft per 1,000 sq ft (≈370 is right), the sheet count left out waste, and the room table didn’t match the guide’s own formulas. All rebuilt.
   • Backsplash guide: a 30 sq ft backsplash needs about 265 3×6″ tiles, not 33–37 (that figure was square feet of tile).
   • Siding guide: a vinyl carton covers one or two squares (100 or 200 sq ft); it said “100 sq ft (two squares).”
   • Framing guide: 20 ft of wall at 16″ o.c. is 16 base studs, not 17, matching the Framing Calculator.
   • French drain guide: a 12″ × 18″ trench holds about 1.5 cu ft of stone per foot before the pipe is subtracted (about 1.4 after).
   • Brick Calculator FAQ and methodology: the firebox example said the side walls take 34 firebrick; 2 × ⌈384 ÷ 22.5⌉ is 36, for 90 total (95 with waste). The calculator itself was already right.
   • Framing FAQ and methodology repeated the 17-stud example; corrected to 16.
   • Interior paint guide: a 3′ × 6′8″ door is 20 sq ft, not 20.33.
4. November 12, 2026

   Lap Siding Calculator

   • The DIY setting said it added 5% waste but multiplied instead, so 10% became 10.5%. It now adds 5 percentage points (10% becomes 15%).
   • Adding a gable wall could lower the waste rate when another wall was marked complex. The highest-waste wall now sets the rate.
   • The guide’s quick answer understated siding for a 1,500 sq ft house (it said 175–200 linear feet per wall side; the real total is about 2,600 linear feet).
   • 8″ and 10″ wood bevel boards used 7-1/8″ and 9-1/8″ widths, which belong to a different siding pattern. They now use WWPA’s bevel dressed widths (7-1/4″ and 9-1/4″), so exposures are 6-1/4″ and 8-1/4″ (slightly fewer boards than before).
   • LP SmartSide exposures now use LP’s actual board widths (7.84″ or 7 13/16″ for an 8″ nominal board) everywhere on the page, and results include a step-by-step “Show the math” panel.
5. November 7, 2026

   Cost guides

   • Removed adjustment multipliers that had no published source from several cost pages. Those factors are now described in words with no number attached.
6. October 29, 2026

   Countertop Calculator

   • The calculator guessed the kitchen layout from the number of runs, so separate straight runs could get a phantom corner deduction and extra seams. You now choose the layout directly, and slab counts check that pieces physically fit on a slab, not just total area.
7. September 22, 2026

   Full calculator sweep

   • Ran every calculator page through an automated browser check (loads, produces a result, no broken values, one heading, complete metadata). Two calculators silently did nothing when a field was blank; they now name the missing field.
   • An EPA lead-paint fine figure on four pages was decades out of date. It is now worded as adjusted annually for inflation.
8. September 18, 2026

   Code-table re-verification

   • Re-checked roughly 1,150 code-table cells and 60 rules against the published code text. Fixed jack-stud counts, masonry reinforcement spacing for high seismic categories, and wind-exposure handling in the framing calculators.
9. September 17, 2026

   Code-table audit

   • Framing: 72 of 249 header-span values allowed longer spans than the IRC permits, and the subfloor joist table exceeded code spans in 14 of 16 cells. Both now use the 2021 IRC tables (R602.7(1)/(2) and R502.3.1(1)) and name the table next to the result.
   • Gutters: the downspout count could come out at 18–35 for an ordinary house. Sizing now follows the UPC and IPC storm-drainage tables.
   • Septic tanks: sizes now come from each state’s own rule text. Florida had been overstated for every home size.