Enter your semi-major axis (a), semi-minor axis (b), and height — each can use a different unit
Enter your 3 measurements
Semi-Major (a)
Half the longest diameter
Semi-Minor (b)
Half the shortest diameter
Height (h)
Vertical depth of tank
Fill Level
100%
25%50%75%Full
⚠ Please enter a valid number (greater than zero) for all three dimensions.
WaterTankCalculator.com — Oval/Elliptical Tank Results
Your Results
Total Volume
—
Litres (L)
Cubic Metres
— m³
US Gallons
— gal
UK/Imperial Gallons
— gal
Cubic Feet
— ft³
Cubic Inches
— in³
Water Weight
— kg
Weight (lbs)
— lbs
Fill Level
— %
Visual Fill Level
📖 How To Use
How to Use This Calculator
Getting your oval or elliptical tank's exact capacity takes under 30 seconds:
Measure the two horizontal axes
Measure the longest internal diameter of the ellipse and divide by 2 — this is the semi-major axis (a). Then measure the shortest internal diameter and divide by 2 — this is the semi-minor axis (b). Always measure from the inside of the tank.
Measure the height
Measure the internal vertical depth from the tank floor to the maximum fill point (usually the overflow). For a horizontal elliptical tank lying on its side, enter the internal length of the tank here instead — a and b then describe the oval end.
Choose your unit per field
Each field has its own unit selector — ft, in, cm, m, or mm. You can mix units across fields; the calculator converts everything internally before computing.
Set the fill level (optional)
Drag the slider to calculate volume at partial fill — for example 75% if you want to know how much water is in the tank right now. For an upright oval tank, the percentage of depth equals the percentage of volume. For a tank lying on its side it does not — the note under the results shows the depth-to-volume relationship for that case. Leave at 100% for maximum capacity.
Hit Calculate
Results appear instantly — litres, gallons, cubic metres, weight in kg and lbs. Use the copy or print buttons to save your results.
Tip: If you only have the full width measurements, simply divide each by 2 to get the semi-axes. A 72 in × 48 in oval has a = 36 in and b = 24 in; a 120 cm × 80 cm oval has a = 60 cm and b = 40 cm.
📐 The Formula
Oval / Elliptical Tank Volume Formula
An elliptical (oval) cross-section is described by two semi-axes. The volume of a vertical elliptical cylinder is:
Volume = π × a × b × h
where a = semi-major axis (half the longest width)
b = semi-minor axis (half the shortest width)
h = height (depth) of tank
Litres (cm inputs): V (L) = (π × a × b × h) ÷ 1,000
Cubic metres (m inputs): V (m³) = π × a × b × h
Cubic feet (ft inputs): V (ft³) = π × a × b × h
US gallons: ft³ × 7.48, or in³ ÷ 231
This calculator converts all units to centimetres internally before computing, so mixed-unit inputs always produce correct results. The value of π used is 3.14159265358979.
Why π × a × b?
The area of an ellipse is π × a × b. For a circle (where a = b = radius), this simplifies to π × r², which is the familiar circle area formula. Multiplying the ellipse area by height gives the tank volume — the same principle as a cylindrical tank, just with an elliptical cross-section.
Some "oval" tanks are really a race-track shape — two straight sides joined by half-circles. For those, the cross-section is (π × r² + 2r × straight length), with r the radius of the rounded ends; the ellipse formula would understate them slightly.
Unit Conversion Reference
Unit
Equals Litres
From Litres
1 Litre (L)
1 L
× 1
1 Cubic Metre (m³)
1,000 L
÷ 1,000
1 US Gallon
3.785 L
÷ 3.785
1 UK Gallon
4.546 L
÷ 4.546
1 Cubic Foot (ft³)
28.317 L
÷ 28.317
1 Cubic Inch (in³)
0.01639 L
× 61.024
Water Weight
1 litre of fresh water weighs about 1 kg (2.2046 lb), and 1 US gallon about 8.34 lb. The exact figure is 0.998 kg per litre at 20 °C (68 °F), so 1 kg per litre is the standard rounding used for load estimates. In US units, 1 cubic foot = 7.48 US gallons and 1 US gallon = 231 cubic inches.
Common oval tank dimensions and their approximate volumes (calculated with V = π × a × b × h):
Nominal Cap.
a × b × h (cm)
Litres
US Gal
Full Weight
250 L
40 × 25 × 80
251
66
251 kg
500 L
55 × 35 × 83
502
133
502 kg
750 L
65 × 40 × 91
743
196
743 kg
1,000 L
70 × 45 × 101
999
264
999 kg
1,500 L
80 × 55 × 108
1,493
394
1,493 kg
2,000 L
95 × 60 × 111
1,988
525
1,988 kg
5,000 L
130 × 85 × 143
4,964
1,311
4,964 kg
10,000 L
180 × 115 × 153
9,950
2,628
9,950 kg
Note: a and b are semi-axes (half the diameter). Actual manufacturer dimensions vary — always verify against the supplier's data sheet.
🏗️
Vertical Oval Tanks
Stand upright with the ellipse as the horizontal cross-section. Common for space-efficient rooftop and basement installations where a round tank won't fit.
🚚
Horizontal Elliptical Tanks
Lie on their side — common on tanker trucks and fuel oil tanks. The elliptical cross-section lowers the centre of gravity for stability. Same formula applies, with the tank's length in place of height.
🏠
Underground Oval Cisterns
Buried horizontal elliptical cisterns make efficient use of underground space. Measuring semi-axes from the inspection hatch is easiest with a tape measure.
⚠️ Common Mistakes
Common Measurement Mistakes for Oval Tanks
1. Confusing Diameter with Semi-Axis
The semi-major axis (a) is half the longest diameter, and the semi-minor axis (b) is half the shortest diameter. If you enter the full diameters instead of the semi-axes, the calculator will give you a result 4× larger than the real volume. Always halve your diameter measurements first.
2. External Instead of Internal Dimensions
Tank walls on oval polyethylene tanks are typically 8–15 mm (about ⅓–⅝ in) thick. Measuring externally overstates the actual storage volume by a few percent. Always measure from the inside of the tank.
3. Using a Circular Formula
If you only have one diameter measurement, do not use V = π × r² × h. An oval tank requires two independent axis measurements. If the cross-section is truly circular (a = b), use our Cylindrical Tank Calculator instead.
4. Forgetting Fill Headspace
A water tank can only fill to its overflow, which sits below the top, and fuel and chemical tanks are often filled to only about 90–95% to leave room for expansion. Measure the height to the overflow, or use the fill slider to plan for realistic operating levels rather than 100% of the geometric volume.
5. Measuring Horizontal Tanks Incorrectly
For a horizontal elliptical tank, the "height" input should be the internal horizontal length of the tank (not the vertical height of the ellipse). The semi-axes describe the cross-sectional ellipse at any point along the tank's length.
🧮 Worked Example
Worked example: an oval tank 72 in × 48 in × 60 in
An upright oval tank measures 72 in by 48 in across inside, and holds water 60 in deep at the overflow.
Semi-axes: a = 72 ÷ 2 = 36 in, b = 48 ÷ 2 = 24 in.
Volume: π × 36 × 24 × 60 = 162,860 in³.
In gallons: 162,860 ÷ 231 = 705 US gallons (2,669 L, 587 UK gallons).
Weight of the water: 705 × 8.34 ≈ 5,880 lb (2,670 kg).
Entering the full widths (72 and 48) by mistake would give 2,820 gallons — four times too much, which is the most common error with this formula.
❓ FAQ
Frequently Asked Questions
How do I calculate the volume of an oval or elliptical water tank?
Use the formula V = π × a × b × h, where a is the semi-major axis (half the longest diameter), b is the semi-minor axis (half the shortest diameter), and h is the tank height. Divide the result in cm³ by 1,000 to get litres, or divide cubic inches by 231 for US gallons. This calculator handles all conversions automatically.
What is the difference between an oval tank and an elliptical tank?
In common usage, the terms are interchangeable for water tanks. Mathematically, a true ellipse is precisely defined by its two semi-axes (a and b), whereas "oval" is a looser term for any egg-like shape. For calculating tank volume, the ellipse formula (V = π × a × b × h) is used for both. It is exact for a true ellipse and close for most oval tanks; for a race-track shape with straight sides, use (π × r² + 2r × straight length) × height instead.
How do I find the semi-major and semi-minor axes of my oval tank?
Measure the longest internal diameter of the oval cross-section (this gives you 2a, so divide by 2 for a). Then measure the shortest internal diameter (this gives you 2b, so divide by 2 for b). Both measurements should be taken inside the tank wall. For a tank labeled "120 cm × 80 cm oval," a = 60 cm and b = 40 cm.
Can I use this calculator for horizontal elliptical tanks?
Yes. For a horizontal tank, the semi-axes (a and b) describe the elliptical cross-section perpendicular to the tank's length. The "height" input should be the internal horizontal length of the tank (the distance from one end to the other). The formula is the same: V = π × a × b × length.
What is the difference between US gallons and UK gallons?
A US gallon equals 3.785 litres; a UK (Imperial) gallon equals 4.546 litres — about 20% larger. This calculator shows both simultaneously so you can use whichever is relevant to your region.
How heavy is a full oval water tank?
Water weighs about 1 kg per litre, or 8.34 lb per US gallon. A 1,000-litre (264-gallon) oval tank carries about 1,000 kg (2,200 lb) of water plus the tank shell (typically 30–70 kg for polyethylene). Always confirm your roof or floor structure can handle the total weight — use our Water Tank Weight Calculator for precise load figures, and consult a structural engineer before rooftop installation.
Why choose an oval tank instead of a round or rectangular tank?
Oval tanks offer a practical middle ground: they fit into tight rectangular spaces better than circular tanks, while avoiding the stress concentrations at corners that rectangular tanks experience under water pressure. They are popular for rooftop installations with limited clearance, underground cisterns, and tanker trucks where a lower centre of gravity improves stability.
Can I use this calculator for liquids other than water?
Yes — the volume results apply to any liquid. Only the weight estimate assumes fresh water (1 kg/L). For other liquids, multiply the litre result by that liquid's density: diesel ≈ 0.85 kg/L, milk ≈ 1.03 kg/L, cooking oil ≈ 0.92 kg/L, seawater ≈ 1.025 kg/L.
✅ Sources & Review
Sources and review
The formula is standard geometry; the conversion factors and figures on this page come from these references:
USGS Water Science School — Water Density: about 1 kg per litre (62.4 lb/ft³, 8.34 lb per US gallon).
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
