Gallons per Square Foot Calculator uses area and applied depth to find gal/sq ft, total gallons, sq ft/gal coverage, wet film thickness, 5-gal pail coverage, and equivalent liquid volume.
Gallons per Square Foot: Why This Number Matters More Than “Total Gallons”
Most paint, sealant, epoxy, and coating product labels list a coverage rate — “covers up to 250 sq ft per gallon” — but that number is usually based on a manufacturer’s ideal film thickness, not the depth your project actually needs. This calculator flips the problem around: instead of guessing how far a gallon will stretch, you tell it the area and the depth (or wet film thickness) you’re applying, and it tells you exactly how many gallons (or liters, if you’re working metric) that combination requires. It’s built for anyone pouring resin floors, applying thick coatings, sealing concrete, or estimating bulk liquid for a slab — situations where “depth” is a real, measurable number rather than a marketing footnote.
How the Numbers Are Worked Out
The tool runs in two unit systems and two area-entry modes, and the math underneath changes slightly depending on which combination you pick.
US units (gallons, feet, inches): Area is converted to square inches (sq ft x 144), then multiplied by the depth in inches to get a volume in cubic inches. Dividing by 231 — the number of cubic inches in a US liquid gallon — gives total gallons. The headline “gallons per square foot” figure comes from the same relationship run per unit area: (depth in inches x 144) / 231. The depth is also converted to mils (thousandths of an inch) for the wet film thickness reading, since that’s the unit most coating manufacturers use on their data sheets.
Metric units (liters, meters, millimeters): This one’s simpler by design — at 1 mm of depth, 1 square meter holds exactly 1 liter. So liters per square meter equals the depth in millimeters, and total liters is just area x depth. Everything else (cubic meters, cubic feet, US gallons) is derived from that liter figure using standard conversion factors.
Area entry — rectangle vs. direct area: If you know the dimensions, switch to rectangle mode and enter length and width; the tool multiplies them for you. If you’ve already measured an irregular space or have a total square footage from a takeoff, switch to area mode and skip straight to entering that number — the length/width fields disappear and the depth field expands to fill the row.
Packaging breakdown: Once total volume is known, the calculator divides it by standard container sizes — 5-gallon pails and 55-gallon drums in US mode, 20-liter pails and 200-liter drums in metric mode — giving both the exact decimal count and a rounded-up “pails needed” figure for ordering.
The Formulas Behind the Numbers
US Units (Gallons / Feet / Inches)
Area (sq ft) = Length x Width [rectangle mode]
OR entered directly [area mode]
Area (sq in) = Area (sq ft) x 144
Volume (cu in) = Area (sq in) x Depth (in)
Total Gallons = Volume (cu in) / 231
Gallons per Sq Ft = (Depth (in) x 144) / 231
Sq Ft per Gallon = 1 / Gallons per Sq Ft
Wet Film Thickness (mils) = Depth (in) x 1000
Cubic Feet = Volume (cu in) / 1728
Cubic Yards = Cubic Feet / 27
Liters = Total Gallons x 3.78541
5-Gal Pails (exact) = Total Gallons / 5
55-Gal Drums (exact) = Total Gallons / 55
Pails Needed = ROUNDUP(5-Gal Pails exact)Metric Units (Liters / Meters / Millimeters)
Area (sq m) = Length x Width [rectangle mode]
OR entered directly [area mode]
Total Liters = Area (sq m) x Depth (mm)
Liters per Sq M = Depth (mm)
Sq M per Liter = 1 / Liters per Sq M
Wet Film Thickness (mm) = Depth (mm) [same value, no conversion]
Cubic Meters = Total Liters / 1000
Cubic Feet = Cubic Meters x 35.3147
Gallons = Total Liters / 3.78541
20-L Pails (exact) = Total Liters / 20
200-L Drums (exact) = Total Liters / 200
Pails Needed = ROUNDUP(20-L Pails exact)Worked Example: Epoxy Topcoat on a Garage Floor
Say you’re coating a 22 ft x 24 ft garage floor (528 sq ft) with a topcoat at 15 mils wet film thickness. Fifteen mils is 0.015 inches, so that’s the depth value to enter.
- Coverage rate: (0.015 x 144) / 231 ~ 0.00935 gallons per sq ft
- Total volume needed: 528 sq ft x 0.00935 ~ 4.93 gallons
- Coverage per gallon: roughly 107 sq ft/gallon at this thickness
- 5-gallon pail coverage: about 535 sq ft — so one pail is just barely enough, with almost nothing left for touch-ups
- Packaging estimate: 0.99 exact pails, rounding up to 1 pail
That last line is the kind of result worth double-checking on the job. The calculator rounds up mathematically, but 0.99 pails leaves zero margin for a second coat, edge buildup, or spills. Most contractors would order a second pail anyway — the tool gives you the honest number; the buffer is on you.
A Common Mistake: Mixing Up “Depth” and “Wet Film Thickness”
In US mode, the depth field is labeled in inches, but most coating spec sheets quote film thickness in mils — and 1 inch equals 1,000 mils, not 100 or 10. Entering “15” into the depth field thinking it means 15 mils will actually calculate for 15 inches of material, producing a volume figure roughly 1,000 times too large.
The calculator does convert correctly internally (depth in inches x 1000 = mils, shown in the Coverage Rate card), but only if the depth value you typed was genuinely in inches to begin with. If your spec sheet says “12-15 mils DFT,” convert to inches first (0.012-0.015) before entering it, or work the conversion backward from the mils figure the tool displays after a first pass.
Frequently Asked Questions
What happens if I enter 0 or leave a field blank?
The calculator requires depth to be a positive number greater than zero in both modes, and requires length and width (rectangle mode) or area (area mode) to also be positive. If any required field is zero, blank, or negative, the results panel clears and displays a prompt asking for valid positive numbers instead of showing a calculation.
Does switching between US and metric units convert my existing numbers?
No — switching unit systems or modes resets the length, width, area, and depth fields back to their defaults (10 x 10, area 100, depth 1.000) rather than converting your entered values. This is intentional: a depth of “1” means 1 inch in US mode but 1 millimeter in metric mode, and those aren’t interchangeable, so the tool avoids silently carrying over a number that would now mean something completely different.
Why does the metric version skip the “cubic yards” and “mils” outputs?
Those units don’t fit naturally into a metric workflow. In metric mode, the equivalent volume card shows cubic meters, cubic feet, and US gallons instead, and the wet film thickness is reported directly in millimeters rather than mils, since mils are a US/imperial measurement.
Why is the “5-Gal Pail Coverage” or “20-L Pail Coverage” figure useful if I already have my total gallons?
It answers a different question: not “how much do I need in total,” but “how much area does one container of this product cover at my chosen depth.” That’s the number you compare directly against your room or slab size to figure out if one pail will get you through a section, a coat, or the whole job before you open a second container.
Does the calculator account for waste, spillage, or multiple coats?
No. Every figure assumes 100% efficient application at a single, uniform depth across the entered area. The note shown alongside results flags that contractors typically build in a 5-10% waste allowance depending on surface porosity and application method — that adjustment isn’t applied automatically and needs to be factored in separately, especially for porous substrates like broom-finished concrete that soak up more material than the calculated depth suggests.
If I’m doing multiple coats, do I just multiply the result?
You can, but only if every coat is the same depth. Enter the depth for a single coat, note the total volume, then multiply by the number of coats — but if a primer or build coat is applied at a different thickness than the topcoat, run the calculator separately for each and add the totals, since the tool only calculates one depth value per pass.