How we compute costs
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A cost estimate is only as good as the method behind it. This page documents ours in full — the formulas, the data sources, the update schedule, and the limitations. If any number on this site can't be traced back to something on this page, that's a bug: tell us.
On this page
The paint calculator formula
wall area = 2 × (length + width) × height − doors×21 − windows×15
paint gallons = wall area × coats ÷ (brand coverage × surface factor)
primer gallons = wall area ÷ 400 (only when priming)
buy gallons = ceil(paint gallons × 1.10) ← 10% waste, rounded up
paint cost = buy gallons × price per gallon
materials = paint cost + primer cost + supplies
labor = wall area × labor rate (pro jobs only; $0 for DIY)
total = materials + labor
planning range = total × 0.75 … total × 1.25 ← ±25%: regional + condition variance- Doors and windows subtract standard openings: 21 sq ft per door (3×7 ft), 15 sq ft per window (3×5 ft).
- Surface factors adjust manufacturer coverage for reality: interior 1.0, ceilings 0.9, exterior 0.8 — rough siding and stucco absorb more paint than smooth drywall.
- Waste defaults to 10% for spillage, touch-ups, and the paint left in rollers and trays.
- Coats: 2 is the default because virtually all manufacturer guidance and pro practice assumes two finish coats; primer is added for new drywall, bare surfaces, or dramatic color changes.
- The ±25% planning range is a stated convention, not a measurement: real bids move with metro labor rates ($1–$6/sq ft nationally), surface condition, and prep scope. We show the range next to every total so a single number never pretends to be more precise than the inputs allow.
Price data sources
Last verified: September 23, 2026
| Data point | Value used | Source | Checked |
|---|---|---|---|
| Behr Premium Plus, 1 gal (interior) | $43.98 | Home Depot product listing #303901388 | Sept 2026 |
| Behr Premium Plus, 5 gal | $196.00 ($39.20/gal) | Home Depot product listing #202761748 | Sept 2026 |
| Behr coverage | 400 sq ft/gal | Manufacturer spec on Home Depot listing | Sept 2026 |
| Sherwin-Williams SuperPaint, 1 gal | $70 (range $60–$77) | MSRP reporting, engineerfix.com; lightmenpainting.com | Sept 2026 |
| Benjamin Moore Regal Select, 1 gal | $74 (range $70–$78) | Ace Hardware listings; O-Gee Paint, Miami | Sept 2026 |
| Primer (Zinsser Bulls Eye 1-2-3), 1 gal | $30 | Ace Hardware ($29.99); Home Depot 2-gal $46.98 | Sept 2026 |
| Pro labor, per sq ft (paintable area) | $1–$6, default $2.50 | HomeGuide & HomeAdvisor 2026 cost guides | Sept 2026 |
| Pro labor, hourly | $25–$75 | HomeGuide 2026 | Sept 2026 |
| Labor share of total project | 70–85% | digitalestimating.com 2026 guide | Sept 2026 |
Where a source gives a range, our calculator default uses the midpoint or the most commonly listed single price, and the page states the full range. Sherwin-Williams runs frequent 30–40% sales; our default uses near-list price and notes the sale effect. Every price is user-overridable in the calculator — local reality beats our defaults.
How we handle labor
Painting labor is quoted three ways in the US market: per square foot of paintable area ($1–$6), per hour ($25–$75), or per room/day. Our calculator uses per-square-foot of paintable area because it's the only basis that scales cleanly from a room to a whole house — and we label the basis explicitly so you're never comparing our number to a quote computed a different way.
Default rate is $2.50/sq ft: the middle of the standard-residential band. High cost-of-living metros and specialty work (cabinets, murals, heavy prep) run higher; straightforward repaints in lower-cost markets run lower. Adjust the rate to match quotes you're actually receiving — that's the right calibration.
Why cost estimates disagree across the web
If you've ever seen two calculators give wildly different numbers for the same project, you're not imagining it. For one well-known query — the cost of a new deck — a major home site's calculator averages $16,007–$17,692 while another respected calculator puts the same project at $3,560–$5,120. Neither site explains the gap. Here's what usually causes it:
- Different scopes. One estimate includes demolition, permits, railings, and stairs; the other assumes a bare-bones platform. Neither says so. Our fix: every estimate on this site lists what's included and what isn't.
- Different units. Painting quotes come per paintable square foot, per floor square foot, per room, or per day — a $3/sq ft floor-area quote is roughly a $1/sq ft wall-area quote. Our fix: we label the basis on every number.
- Different regions and dates. Labor in San Francisco vs. rural Ohio differs 2–3×, and a 2021 price table is not a 2026 price table. Our fix: dated sources on every figure, and the ±25% planning range shown next to every total.
- Different business models. Lead-generation sites are incentivized to show numbers that make you pick up the phone, in either direction. Our fix: we sell nothing and gate nothing — the calculator's math is the product.
When you compare any two estimates — ours against a contractor's, or ours against another site — normalize them first: same scope, same unit, same year, same region. Most "disagreements" evaporate once you do.
Update cadence
- Paint prices: re-checked quarterly (January, April, July, October), or sooner if we detect a major price move.
- Labor ranges: re-checked semi-annually against published cost guides.
- Calculator logic: reviewed whenever user feedback or our own testing finds a flaw.
- Every data table on the site carries its check date. If a date is older than the cadence above, the data is stale and we say so.
Limitations — read this
- Our numbers are planning estimates, not quotes. Surface condition, regional pricing, and contractor overhead move real bids ±30% or more.
- We track US national pricing. Costs in high-cost metros (NYC, SF, Boston) routinely run 30–50% above our defaults.
- Coverage rates are manufacturer specs under ideal conditions. Textured walls, porous surfaces, and dark colors need more paint — the waste factor and surface adjustments only partly capture this.
- We don't have paid keyword or pricing datasets; where we report third-party figures we name the source and date, and mark anything we couldn't verify.