How to Calculate Rectangular Tank Capacity
Measure the clear internal length, width and vertical height. Choose the corresponding unit and enter all three measurements. The tank must have a level, rectangular bottom and straight vertical sides.
For a working liquid level, enter the depth above the inside bottom. A blank depth returns full capacity only; zero depth explicitly means the tank is empty.
Rectangular Tank Volume Formula
Full volume = length × width × height
Liquid volume = length × width × liquid depth
Use the same length unit for every dimension. The resulting volume is in that unit cubed.
Calculating Capacity in Litres
Multiply cubic meters by 1,000. If all measurements are in centimeters, divide the product by 1,000 instead.
Calculating Capacity in US Gallons
When measurements are in inches, divide length × width × height by 231. One US liquid gallon is exactly 231 cubic inches. Imperial gallons use a different conversion.
Partial Water Level Calculations
A rectangular tank's base area stays constant at every height. A tank filled to 60% of its inside height contains 60% of its geometric capacity. Remaining capacity is the full volume minus current liquid volume.
That proportional relationship does not apply to a cylinder lying on its side. Use the horizontal cylinder liquid-volume calculator for that shape, or the tank volume calculator to compare all three supported shapes.
Worked Examples
Metric: a 1,000-liter tank
An internal length of 2 m, width of 1 m and height of 0.5 m gives 2 × 1 × 0.5 = 1 m³, or 1,000 liters. At 0.3 m liquid depth, it holds 600 liters and has 400 liters of geometric space remaining.
US customary: dimensions in inches
A rectangular tank measuring 48 in × 24 in × 20 in has 23,040 cubic inches of capacity. Dividing by 231 gives 99.740 US gallons (377.558 liters). At 12 in liquid depth, it holds 59.844 US gallons, or 60% of its full volume.
Common Measurement Mistakes
Outside dimensions include wall thickness. Measure inside when possible; if thickness is uniform, subtract both walls from each outside dimension that includes two walls. Do not automatically subtract two wall thicknesses from the height of an open-top tank.
Nominal tank sizes may differ from usable liquid capacity. Freeboard, internal fittings, baffles, rounded corners and a sloping base can reduce the available space. This tool does not estimate those adjustments.
Frequently Asked Questions
Does a square tank use the same formula?
Yes. A square base simply has equal length and width. Multiply those by the internal height.
Why is my result different from the tank label?
A label may give a nominal size or recommended working capacity. The calculator uses only the ideal space implied by your internal measurements.
Does this work for a sloping tank bottom?
No. The rectangular formula assumes a level base, straight vertical walls and a constant horizontal cross-section.
Methodology & Limitations
Tank Volume Lab calculates ideal geometry in cubic meters, then converts the result using fixed unit definitions. Cylinders use circular area; horizontal partial fills use circular-segment area. Rectangular tanks use base area times height. See the circular-segment formula and NIST unit conversion guidance.
Results are estimates based on ideal internal geometry. Real usable capacity can differ because of wall thickness, tank fittings, baffles, dished ends, freeboard and calibration requirements. These estimates do not replace certified measurement for industrial, safety or financial purposes.