Flat Bottom Tank Volume Calculator: How It Works

This calculator works out the volume of a flat-bottomed tank (cylinder, rectangular box, or horizontal cylinder with flat ends) from the dimensions you enter, and returns the volume in litres and US gallons. Enter the tank’s length, width, and height (or diameter and length for a cylinder), and the calculator gives the total volume and, if you provide a liquid depth, the volume of liquid currently in the tank.

Formula

The volume of a flat-bottomed tank depends on its shape. For a rectangular tank (a box):

V = L × W × H

where V is volume (cubic metres), L is length (m), W is width (m), and H is height (m). Multiply by 1000 to get litres, or by 264.172 to get US gallons.

For a vertical cylinder (flat bottom, flat top):

V = π × r² × H

where r is the radius (half the diameter) and H is the height. For a horizontal cylinder (flat ends), the total volume is the same formula, but the liquid volume at a given depth is more complex because the cross-section is a circular segment. The calculator above uses the rectangular and vertical-cylinder formulas for total volume, and for the horizontal cylinder it computes the liquid volume using the segment area formula:

A_seg = r² × cos⁻¹((r−h)/r) − (r−h) × √(2rh − h²)

where h is the liquid depth. Multiply the segment area by the tank length to get liquid volume.

Worked Example

Using the default values (rectangular tank, length 2 m, width 1 m, height 1 m, liquid depth 0.5 m):

1. Total volume = 2 × 1 × 1 = 2 m³.
2. Convert to litres: 2 × 1000 = 2000 L.
3. Convert to US gallons: 2000 / 3.78541 = 528.34 gal.
4. Liquid volume = 2 × 1 × 0.5 = 1 m³ = 1000 L.

The calculator shows: total volume 2000.00 L, liquid volume 1000.00 L, and total in gallons 528.34 gal.

When to Use This Calculator

Use this tool for any flat-bottomed tank where you need a quick volume estimate: a fish tank, a rainwater storage tank, a fuel tank, a chemical mixing tank, or a septic tank. It is especially handy when you want to know how much liquid is currently in a partially filled horizontal cylinder—a common situation for fuel or water storage on farms and in industry.

For a horizontal cylinder, the liquid volume is not a simple linear function of depth; at half depth you get half the volume, but at a quarter depth you get less than a quarter. This calculator handles that nonlinearity for you.

How to Choose the Right Tank Shape

• Rectangular: Use for aquariums, plastic storage totes, concrete sumps, or any tank with straight sides and a flat bottom.
• Vertical cylinder: Use for upright drums, pressure vessels, or water towers. The liquid surface is always a full circle, so the liquid volume is simply proportional to depth.
• Horizontal cylinder: Use for fuel tanks on trucks, propane tanks (though those are usually elliptical), or long storage tanks lying on their side. The liquid volume requires the segment calculation.

If your tank has a sloped bottom, dished ends, or a conical top, this calculator will over- or under-estimate. For those, you need a more advanced calculator or a manufacturer’s strapping table.

Common Mistakes and How to Avoid Them

• Unit mix-ups: The calculator expects metres. If you measure in centimetres, convert to metres by dividing by 100. If you measure in feet, multiply by 0.3048. A 120 cm × 60 cm × 50 cm aquarium is 1.2 × 0.6 × 0.5 = 0.36 m³ = 360 L, not 360,000 L.
• Diameter vs. radius: For cylinders, the calculator asks for diameter (or height, which is the same for a vertical cylinder). If you enter the radius as the diameter, you’ll get a quarter of the true volume.
• Liquid depth vs. tank height: For a horizontal cylinder, the liquid depth is measured from the bottom of the tank, not from the top. If you measure the empty space at the top, subtract that from the diameter.
• Forgetting the flat bottom: This calculator is only for flat bottoms. If your tank has a dished or conical bottom, the volume is different—often 5–10% more.

Limitations of the Formula

The formulas assume perfect geometry: straight sides, flat ends, and a uniform cross-section. Real tanks may have rounded corners, internal baffles, or slight bulges. The calculator also does not account for the thickness of the tank walls—if you need the internal volume, measure the inside dimensions, not the outside. For a steel tank with 6 mm walls, the difference is small, but for a thick plastic tank it can matter.

Also, the liquid volume calculation assumes the tank is level. If it sits at an angle, the liquid surface is not horizontal relative to the tank, and the volume will be different. For tilted tanks, you need a more complex calculator or a calibration chart.

Frequently Asked Questions

How do I calculate the volume of a rectangular tank in litres?

Measure the length, width, and height in metres, multiply them together to get cubic metres, then multiply by 1000. For example, a 2 m × 1 m × 1 m tank holds 2000 litres.

How do I calculate the volume of a horizontal cylinder tank?

Use the formula for the circular segment area: A = r² × cos⁻¹((r−h)/r) − (r−h) × √(2rh − h²), where r is the radius and h is the liquid depth. Multiply by the tank length. The calculator above does this automatically when you select “Horizontal cylinder”.

What is the difference between litres and US gallons?

One US gallon is exactly 3.785411784 litres. To convert litres to US gallons, divide by 3.78541. To convert to imperial gallons (UK), divide by 4.54609. The calculator uses US gallons, which are common in the United States and some other countries.

Why is the liquid volume not proportional to depth in a horizontal cylinder?

Because the cross-section is a circle, not a rectangle. Near the bottom, a small increase in depth adds a large area; near the top, the same increase adds a small area. The relationship is nonlinear, which is why a simple “depth divided by diameter times total volume” is inaccurate.

Can I use this calculator for a tank with a conical bottom?

No. This calculator assumes a flat bottom. For a conical bottom, you would need to add the volume of the cone (1/3 × π × r² × h_cone) to the volume of the cylindrical part. Some tanks have a sloped bottom for drainage; those require a different formula.

What if my tank is not completely full?

Enter the liquid depth in the “Liquid depth” field. For rectangular and vertical cylinder tanks, the liquid volume is simply the total volume times (liquid depth / tank height). For horizontal cylinders, the calculator uses the segment formula to give an accurate reading.

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in shape | Tank shape | Rectangular=1, Vertical cylinder=2, Horizontal cylinder=3 | 1 | 0
in len | Length | m | 2 | 0
in wid | Width (rectangular) | m | 1 | 0
in hei | Height / Diameter | m | 1 | 0
in liq | Liquid depth (optional) | m | 0.5 | 0
out vol_total | Total volume | L | shape * len * wid * hei * 1000 | 2
out vol_liq | Liquid volume | L | shape * len * wid * liq * 1000 | 2
out gal_total | Total volume (US gal) | gal | vol_total / 3.78541 | 2