📖 How To Use
How to Use This Calculator
Getting your oval or elliptical tank's exact capacity takes under 30 seconds:
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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.
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Measure the height
Measure the internal vertical depth from the tank floor to the maximum fill point. For horizontal elliptical tanks, "height" is the vertical internal diameter.
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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.
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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. Leave at 100% for maximum capacity.
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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 diameter measurements, simply divide each by 2 to get the semi-axes. 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
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.
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 = 1 kg (or 2.2046 lbs). This standard is used for structural load calculations worldwide.
Weight (kg) = Volume (litres) × 1
Weight (lbs) = Volume (litres) × 2.2046
📏 Reference Data
Standard Oval Tank Size Reference
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 | 744 | 196 | 744 kg |
| 1,000 L | 70 × 45 × 101 | 998 | 264 | 998 kg |
| 1,500 L | 80 × 55 × 108 | 1,492 | 394 | 1,492 kg |
| 2,000 L | 95 × 60 × 111 | 1,987 | 525 | 1,987 kg |
| 5,000 L | 130 × 85 × 143 | 4,975 | 1,314 | 4,975 kg |
| 10,000 L | 180 × 115 × 153 | 9,969 | 2,633 | 9,969 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. The elliptical cross-section lowers the centre of gravity for stability. Same formula applies; just swap which axis is 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 thick. Measuring externally overstates the actual storage volume. 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
Most manufacturers recommend leaving 10–15% empty to allow for thermal expansion and prevent overflow. Use the fill slider to plan for realistic operational levels rather than 100% full capacity.
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.
❓ 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. 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, and it is accurate for all standard oval-cross-section tanks.
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 1 kg per litre. A 1,000-litre oval tank carries 1,000 kg 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.