Painting Calculator for Accurate Interior & Exterior Estimates

Inputs and outputs

Start with geometry (length, width, height) and subtract openings. Split work by surface: walls, ceilings/soffits, trim, doors, cabinets. Record substrate and method, target coats, primer rules, brand/sheens, color, and any height/access constraints. Scope prep explicitly—masking, patching, caulking, washing/scraping—so hours aren’t hidden in “misc.”

Your outputs should mirror what crews do: fractional gallons per surface, a clear container plan, labor hours by task with setup/cleanup, and any equipment or schedule notes. List the assumptions you want signed off.

Measuring surfaces correctly (formulas + example)

Walls: area = (perimeter × height) − openings. Ceilings: length × width. Doors/trim: count or linear footage as appropriate.

Coats and primer decision rules

Plan extra coats when there is a meaningful color change, deep hues, heavy texture, or prior coverage shortfalls. Use primer for nicotine/smoke, stained repairs, glossy substrates, raw wood/metal, or masonry that calls for it. If you set three coats after seeing the space, record that choice so it stays consistent across the estimate.

Coverage and gallon math

A simple working formula is: gallons = (area ÷ coverage) × coats × (1 + waste). On smooth drywall with brush/roll, 350–400 ft²/gal is common. Waste typically sits between 5–15% based on masking quality and crew familiarity.

Common pitfalls (and quick fixes)

Labor production rates that hold up in the field

Use production rates you can defend: rolling walls around 350–500 ft²/hour, cutting trim 100–150 LF/hour, doors around 3–4 per hour. Ceilings are slower—apply a height/access multiplier and don’t forget setup/cleanup per space. Round to a professional increment (e.g., quarter‑hour) so totals align with how crews clock time.

Scoping prep without guessing

List masking, patching, caulking, and washing/scraping explicitly. Record typical time ranges and flag exceptions early (nicotine remediation, water damage, heavy repairs) so they are priced intentionally, not absorbed.

Worked example (single room)

Prep (typical ranges)

Advanced material cost control

Container planning compares 5‑gal, 1‑gal, and quarts to cover needed gallons at the lowest total cost. Keep colors separate—no cross‑pooling. The effective price per gallon is total plan cost divided by needed gallons, so when you allocate costs back to fractional needs, the sum matches the plan.

Key terms, in plain language

Coverage. How many square feet a gallon reliably paints for your substrate and method.

Waste factor. A reasonable allowance for setup, pour‑off, overlap, and learning curve.

Production rate. The throughput your crews achieve per task (e.g., ft²/hour or doors/hour).

Effective per‑gal. Optimized container plan cost divided by needed gallons; use it to allocate material cost back to fractional needs.

FAQ