Introduction
The Garage Paint Calculator runs two parallel calculations — walls and floors — each with its own area formula and coverage rate. This guide derives both so you can verify the math.
Formula 1: Net Wall Area
Awall = 2 × (L + W) × H − ΣAopenings
Perimeter (2 × (length + width)) times height, minus each opening’s area. Openings:
- Single garage door: ~2.4 m × 2.1 m = 5 m² (8 × 7 ft = 56 ft²)
- Double garage door: ~4.8 m × 2.1 m = 10 m² (16 × 7 ft = 112 ft²)
- Entry door: ~0.9 m × 2 m = 1.8 m² (3 × 6.7 ft = 20 ft²)
- Window: measure individually, typically 1–1.5 m² each
Formula 2: Floor Area
Afloor = L × W
No deductions — coat the entire slab including under where vehicles park.
Formula 3: Wall Paint Gallons
Gwall = ceil(Awall ÷ Cwall × Ncoats)
Cwall ≈ 35 m²/L (375 ft²/gal) for acrylic satin. N = 2 topcoats (plus primer calculated separately at ~23 m²/L, one coat).
Formula 4: Floor Coating Gallons
Gfloor = ceil(Afloor ÷ Cfloor × Ncoats)
Cfloor ≈ 20 m²/L (215 ft²/gal) for 2-part epoxy. Then round to kit sizes — epoxy is sold in kits, not arbitrary gallons.
Worked Derivation
Garage 7 m × 6 m × 2.5 m, double door + entry door + 2 windows. Bare walls (need primer), bare floor.
Walls: perimeter 2 × 13 = 26 m × 2.5 = 65 m². Openings: 10 + 1.8 + 2.5 = 14.3 m². Net: 50.7 m². Paint: 50.7 ÷ 35 = 1.45 L/coat × 2 = 2.9 L → 1 gal. Primer: 50.7 ÷ 23 = 2.2 L → 1 gal.
Floor: 7 × 6 = 42 m². Epoxy: 42 ÷ 20 = 2.1 L/coat × 2 = 4.2 L → 2 gal (or one 2-car kit).
Imperial: walls 546 ft² ÷ 375 = 1.46 × 2 = 2.9 → 1 gal ✓. Floor 452 ft² ÷ 215 = 2.1 × 2 = 4.2 L → 2 gal ✓.
Worked Example 2: 1-Car Garage
A compact single-car garage: 3.7 m × 6 m (12 × 20 ft), 2.4 m (8 ft) ceilings, one single garage door, one entry door, one window.
Walls (metric): perimeter 2 × (3.7 + 6) = 19.4 m × 2.4 = 46.6 m². Openings: door 5.0 + entry 1.8 + window 1.2 = 8.0 m². Net: 38.6 m². Two coats at 35 m²/L: 38.6 ÷ 35 = 1.10 L/coat × 2 = 2.2 L → 1 gallon (3.8 L) with comfortable margin. Primer at 23 m²/L: 38.6 ÷ 23 = 1.68 L → 1 gallon.
Walls (imperial): perimeter 64 ft × 8 = 512 ft². Openings: 56 + 20 + 13 = 89 ft². Net: 423 ft². 423 ÷ 375 = 1.13 gal/coat × 2 = 2.26 → 1 gallon. (The single gallon is a rounding-up convenience: you’re buying ~1.6 gal of actual need in quart+gal combinations, but retail paint rarely justifies a 1.5-gal purchase — one gallon leaves you short, so buy a gallon plus a quart.)
Floor: 3.7 × 6 = 22.2 m² (12 × 20 = 240 ft²). Epoxy at 20 m²/L: 22.2 ÷ 20 = 1.11 L/coat × 2 = 2.2 L → 1 gal. Imperial: 240 ÷ 215 = 1.12 × 2 = 2.24 gal → most 1-car epoxy kits (rated ~250 ft² / 23 m² two-coat) cover this exactly; buy the 1-car kit rather than mixing partial 2-car kit components.
Worked Example 3: 3-Car Garage
A large three-car garage: 9 m × 7 m (30 × 23 ft), 2.7 m (9 ft) ceilings, two garage doors (one double, one single), one entry door, three windows.
Walls (metric): perimeter 2 × (9 + 7) = 32 m × 2.7 = 86.4 m². Openings: 10.0 (double) + 5.0 (single) + 1.8 + 3 × 1.2 (3.6) = 20.4 m². Net: 66.0 m². Paint: 66 ÷ 35 = 1.89 L/coat × 2 = 3.77 L → 1 gallon (3.8 L), just barely — with porous or textured walls, round up to a gallon plus a quart (4.7 L total). Primer: 66 ÷ 23 = 2.87 L → 1 gallon.
Walls (imperial): perimeter 106 ft × 9 = 954 ft². Openings: 112 + 56 + 20 + 3 × 13 (39) = 227 ft². Net: 727 ft². 727 ÷ 375 = 1.94 × 2 = 3.87 gal → 4 gallons. The metric/imperial rounding difference comes from the 35 m²/L vs 375 ft²/gal rates being rounded equivalents — always buy to the ceiling in whichever unit you shopped.
Floor: 9 × 7 = 63 m² (30 × 23 = 690 ft²). Epoxy at 20 m²/L: 63 ÷ 20 = 3.15 L/coat × 2 = 6.3 L → ~1.7 gal. Imperial: 690 ÷ 215 = 3.2 × 2 = 6.4 gal → 1.5 to 2 two-car kits (each rated ~500 ft² / 46 m²). Here the kit-rounding rule bites: buying two 2-car kits leaves significant leftover, but a single 2-car kit plus a 1-car kit is the economical sweet spot if the brand sells both sizes — 500 + 250 = 750 ft² of rated coverage for 690 ft² of need.
Kit-Size Rounding Logic
Epoxy is sold in kits, not bulk gallons, and this changes the math from “calculate precisely” to “find the cheapest kit combination that covers.” The logic:
- Compute exact liquid need using Formula 4 (area ÷ coverage × coats).
- Translate to rated coverage: most manufacturers rate kits by two-coat square footage — a “2-car kit, 500 ft²” means 500 ft² at the manufacturer’s assumed two-coat build. Compare your floor area directly against rated coverage, not against liquid gallons.
- Add the porosity buffer: on bare, unsealed, or rough concrete, effective coverage drops 15–25%. Multiply your area by 1.2 before comparing to rated coverage. A 400 ft² (37 m²) bare slab should be matched against a kit rated for ~480 ft² (45 m²).
- Round up to the next kit: if your buffered area exceeds a kit’s rating, step up to the next size — never plan to stretch a kit thinner than rated. Thin epoxy is the most common cause of premature wear.
- Combine sizes when economical: many brands sell 1-car (~250 ft² / 23 m²) and 2-car (~500 ft² / 46 m²) kits. For 690 ft² (64 m²): one 2-car + one 1-car (750 ft² rated) beats two 2-car kits (1,000 ft² rated) by a wide margin — but confirm both kits are the same product line, since mixing lines voids the system’s chemistry compatibility.
- Account for waste: 2-part epoxy has real waste — material left in mixing buckets, on the brush for cut-in, and the unusable last minutes of each batch. Add 5–10% on top of calculated need before kit-matching.
Wall paint rounding is simpler but still deliberate: paint is sold in quarts (0.95 L), gallons (3.8 L), and 5-gallon buckets (19 L). If you need 2.2 L, buy 1 gallon — not 2 quarts (1.9 L, short) and not 2 gallons (wasteful). If you need 4.5 L, buy 1 gallon + 1 quart (4.75 L). Only buy the 5-gallon bucket if you genuinely need 15+ L; leftover paint stored in a half-empty bucket skins over and degrades.
Metric/Imperial Derivations
Coverage rates are quoted per liter in metric and per gallon in imperial, and they’re not exact conversions of each other — they come from manufacturer data sheets written in each market’s units. Here’s how they relate:
- Wall acrylic satin: 35 m²/L. Converted: 35 × 10.764 ft²/m² ÷ 3.785 L/gal = 375 ft²/gal. This one converts cleanly.
- Wall primer: 23 m²/L → 23 × 10.764 ÷ 3.785 = 250 ft²/gal (often labeled 250–300 ft²/gal by US brands; use the low end for porous surfaces).
- 2-part epoxy: 20 m²/L → 20 × 10.764 ÷ 3.785 = 215 ft²/gal. Note some US labels say 200–250 ft²/gal per coat — the 215 figure assumes a proper build thickness, not a stretched one.
- 1-part concrete paint: 25 m²/L → 268 ft²/gal, commonly rounded to 250–300 ft²/gal on labels.
- Polyaspartic: 18 m²/L → 193 ft²/gal, labeled 175–225 ft²/gal.
The practical rule: compute in the units you’ll shop in. If you’re buying gallons in the US, use ft²/gal rates and divide your ft² area. If you’re buying liters, use m²/L rates. Don’t convert areas then apply the “equivalent” rate — rounding in the published rates introduces small errors, and it’s the shopping-unit math that determines what you actually buy. The Garage Paint Calculator does both natively.
Deriving Opening Deductions
Why do openings matter enough to get their own formula term? A double garage door at 10 m² (112 ft²) is roughly 15–20% of total wall area — skipping the deduction overbuys paint by nearly a fifth. The deduction formula ΣAopenings sums each opening’s width × height. Include garage doors (measure — 2.4 m / 8 ft heights exist and change the area 15%), entry doors, windows, and large vents over ~0.5 m² (5 ft²).
Don’t deduct: outlets, small fixtures, or areas behind removed shelving — rounding noise. And never deduct openings from floor area: the slab continues under the door swing, and coating must run wall-to-wall.
Related Guides
- Complete Guide — full walkthrough.
- Cost Guide — pricing.
- FAQ — quick answers.
Get Your Number
Two formulas, two products, one calculator. The Garage Paint Calculator runs both instantly.
Disclaimer: Use manufacturer coverage rates for your specific products.
