Powerful Square Feet to Gallons Calculator
1 square foot of water, 1 inch deep = 0.623 gallons. Use this base conversion to calculate gallons per square foot for water, paint, or any liquid coating.
Enter dimensions to seelive results and step-by-step calculations.
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Gallons Per Square Foot Calculator
1 square foot of water, 1 inch deep = 0.623 gallons. Use this base conversion to calculate gallons per square foot for water, paint, or any liquid coating.
This calculator converts square feet to gallons (and gallons to square feet) using area, depth, or manufacturer coverage rates — so you get accurate volume for irrigation, pools, sealants, and paint projects without manual math.
How Many Gallons Are in a Square Foot?
A square foot measures area; a gallon measures volume. You cannot convert one to the other without depth (thickness or height).
Formula:
Gallons = Square Feet × Depth (in feet) × 7.48052
Because 1 cubic foot = 7.48052 US liquid gallons:
- 1 sq ft × (1 inch ÷ 12) × 7.48052 = 0.623 gallons
- 1 inch of water over 1 square foot = 0.623 gallons (standard in US irrigation and rainfall math)
- 12 inches (1 foot) deep over 1 sq ft = 7.48 gallons
Gallons Per Square Foot for Water
For water, coatings, or any liquid spread to a known depth, use volume math:
Quick depth reference
| Depth | Gallons per square foot |
|---|---|
| 1 inch | 0.623 gal |
| 2 inches | 1.247 gal |
| 3 inches | 1.87 gal |
| 6 inches | 3.74 gal |
| 12 inches (1 ft) | 7.48 gal |
Example: 1,000 sq ft at 1 inch
1,000 × 0.623 = 623 gallons of water — common for lawn irrigation or 1 inch of rainfall over an area.
Gallons Per Square Foot for Paint and Coatings
Paint and liquid coatings usually list square feet per gallon on the label, not depth. Use coverage rate instead of the 0.623 water rule.
Gallons per sq ft = 1 ÷ (sq ft covered per gallon)
Example: If 1 gallon covers 350 sq ft → 1 ÷ 350 = 0.00285 gallons per square foot.
For 640 sq ft at 350 sq ft/gallon: 640 ÷ 350 = 1.83 gallons (round up 10–15% for texture or porous surfaces).
How to Calculate Gallons Per Square Foot
Follow these steps for any rectangular area:
Step 1 — Area in square feet
Area = Length × Width
Example: 20 ft × 10 ft = 200 sq ft
Step 2 — Depth in feet
If depth is in inches: Depth (ft) = Inches ÷ 12
Example: 3 inches = 0.25 ft
Step 3 — Volume and gallons
Cubic feet = Area × Depth
Gallons = Cubic feet × 7.48052
Example: 200 sq ft × 0.25 ft × 7.48052 = 374 gallons total
Gallons per sq ft = Total gallons ÷ Area → 374 ÷ 200 = 1.87 gal/sq ft
Square Feet to Gallons vs Gallons to Square Feet
Square feet → gallons: Gallons = Sq Ft × Depth (ft) × 7.48052
Gallons → square feet: Square Feet = Gallons ÷ (Depth × 7.48052)
Both need depth (or a paint coverage rate). Without depth, area and volume cannot be swapped.
Real-World Uses
- Irrigation — 1 inch of water per sq ft = 0.623 gal; scale by total lawn or field area
- Pools — length × width × average depth → gallons for pumps and chemicals
- Rainfall — stormwater and runoff estimates from inches of rain over sq ft
- Concrete curing & waterproofing — liquid volume per covered area
- Paint & sealants — gallons from label coverage (sq ft per gallon)
Tips for Accurate Results
- Add 5–10% extra for spillage, uneven spread, or waste
- Porous concrete and textured walls need more material than smooth surfaces
- Confirm AM/PM or 24-hour units only when using time inputs — this tool uses dimensions, not clock time
- For paint, always use the manufacturer’s sq ft per gallon on the can
- Wind and evaporation can reduce effective irrigation volume outdoors
Frequently Asked Questions
How many gallons per square foot of water?
At 1 inch depth, 1 square foot holds 0.623 gallons. At 1 foot depth, 1 square foot holds 7.48 gallons. The exact volume depends on the depth of water over the surface area.
How many gallons per square foot of paint?
It depends on the product. Standard interior paint covers 350–400 sq ft per gallon, meaning each square foot needs roughly 0.0025 to 0.003 gallons. Thicker coatings like epoxy need more — usually one gallon per 200–250 sq ft.
How do I convert square feet to gallons for water?
Multiply square feet by depth in inches, then multiply by 0.623. This gives you US gallons.
What does gallons per square foot per inch mean?
It means the volume of liquid (in gallons) that occupies one square foot of surface area at one inch of depth. The value is always 0.623 gallons per square foot per inch for water.
How many gallons are in a square foot of water at 1 foot depth?
1 cubic foot of water = 7.48052 US gallons. So a 1 sq ft column of water 1 foot deep contains 7.48 gallons.
How many gallons of paint do I need for 500 square feet?
At standard coverage of 400 sq ft per gallon: 500 ÷ 400 = 1.25 gallons. Buy 2 gallons to allow for a second coat and touch-ups.
How many gallons of water in a square inch?
1 square inch, 1 inch deep = 1 cubic inch. Since 1 US gallon = 231 cubic inches, 1 cubic inch = 0.00433 gallons.
What is the formula for gallons per square foot of rainfall?
Gallons = Area (sq ft) × Rainfall depth (inches) × 0.623. For example, 1 inch of rain on 1,000 sq ft = 623 gallons.
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Formulas Used
A = L × W (Area in square feet)
Vft³= A × H = (L × W) × H (Volume in cubic feet)
Vgal= Vft³× 7.48052 (Volume in US gallons)
G = Vgal÷ A (Gallons per square foot)
Common Applications
- Pool & Spa:Calculate volume and chemical dosing rates
- Tank Design:Determine liquid storage capacity per floor area
- Irrigation:Calculate application rates for landscaping
- Construction:Estimate concrete, paint, or coating coverage
- Aquaculture:Design fish tanks and water systems
Conversion Factor
1 cubic foot = 7.48052 US gallons
This conversion factor is exact and widely used in construction, plumbing, and engineering applications.
Example Calculation
Given:Length = 10 ft, Width = 8 ft, Height = 3 ft
Solution:
Area = 10 × 8 = 80 ft²
Volume = 80 × 3 = 240 ft³
Volume in gallons = 240 × 7.48052 = 1,795.32 gallons
Gallons per sq ft = 1,795.32 ÷ 80 = 22.44 gal/ft²