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Paint Calculator

Gallons of paint for a room. Free online Paint Calculator. Calculate paint online — fast, accurate, mobile-friendly, no signup needed.

Gallons (one coat)
1.005714

Derivation

  1. ├── 01Givenl = 12, w = 10, h = 8, coverage = 350
  2. ├── 02Formula2 × e.h × (t+a) / e.coverage
  3. ├── 03Substitute2 × e.8 × (t+a) / e.350
  4. └── 04Compute Gallons (one coat)1.005714
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§01What is

Understanding the Paint Calculator

The Paint Calculator computes Gallons (one coat) from 4 inputs: length (ft), width (ft), height (ft), coverage (ft²/gal). Gallons of paint for a room.

On a construction site, estimates that come in 10% off add up to six-figure overruns. Running the quantities with a calculator instead of a rule-of-thumb gets you closer to the truth with zero extra effort. The Paint Calculator sits in that toolkit — it gallons of paint for a room. Enter your numbers above and the result updates instantly; every step of the math is shown in the Derivation panel so you can see exactly how the answer was reached.

§02The Formula

How it’s calculated

2 × e.h × (t+a) / e.coverage

Where

l
Length (ft)
w
Width (ft)
h
Height (ft)
coverage
Coverage (ft²/gal)
§03Practical Example

Step-by-step walkthrough

Scenario

Apply the formula to a realistic set of inputs: Length (ft) = 12, Width (ft) = 10, Height (ft) = 8, Coverage (ft²/gal) = 350.

  1. 01Start by noting the input — Length (ft): 12.
  2. 02Start by noting the input — Width (ft): 10.
  3. 03Start by noting the input — Height (ft): 8.
  4. 04Start by noting the input — Coverage (ft²/gal): 350.
  5. 05Substitute these values into the formula: 2 × e.h × (t+a) / e.coverage
  6. 06Compute Gallons (one coat): the calculator returns 1.00571.
  7. 07Cross-check the answer by opening the Derivation panel above — every line of math is shown so you can follow the computation end-to-end.
§04Variants

Common Paint Problems

The formula gets rearranged depending on which variable you need. Here are the patterns you’ll run into in the real world — find the one that matches your problem and follow the worked steps.

01 · PATTERN

Length (ft) halved

l = 6 (from 12)

Keep every other input at its default and halve the length (ft). See how gallons (one coat) responds.

  1. 01New Length (ft): 6
  2. 02Baseline Gallons (one coat): 1.00571
  3. 03New Gallons (one coat): 0.731429
  4. 04Gallons (one coat) decreases by 27.3% → use this sensitivity to plan for real-world variation.
02 · PATTERN

Length (ft) doubled

l = 24 (from 12)

Keep every other input at its default and double the length (ft). See how gallons (one coat) responds.

  1. 01New Length (ft): 24
  2. 02Baseline Gallons (one coat): 1.00571
  3. 03New Gallons (one coat): 1.55429
  4. 04Gallons (one coat) increases by 54.5% → use this sensitivity to plan for real-world variation.
03 · PATTERN

Width (ft) halved

w = 5 (from 10)

Keep every other input at its default and halve the width (ft). See how gallons (one coat) responds.

  1. 01New Width (ft): 5
  2. 02Baseline Gallons (one coat): 1.00571
  3. 03New Gallons (one coat): 0.777143
  4. 04Gallons (one coat) decreases by 22.7% → use this sensitivity to plan for real-world variation.
04 · PATTERN

Width (ft) doubled

w = 20 (from 10)

Keep every other input at its default and double the width (ft). See how gallons (one coat) responds.

  1. 01New Width (ft): 20
  2. 02Baseline Gallons (one coat): 1.00571
  3. 03New Gallons (one coat): 1.46286
  4. 04Gallons (one coat) increases by 45.5% → use this sensitivity to plan for real-world variation.
§05FAQ

Frequently asked questions

Yes. The calculator implements the standard formula as documented and returns exact floating-point results. No approximations are used unless noted in the formula.
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