Most standard concrete and plastic water tanks in the Mangawhai area hold 24,000, 25,000 or 30,000 litres. If you are not sure what yours holds, you can work it out in a few minutes with a tape measure: for a round tank, multiply π × radius × radius × water height in metres, then multiply by 1,000 to get litres.
Below is the formula step by step, a worked example, and what one water delivery does to the level in your tank.
Why your tank size matters
Knowing your tank’s capacity helps you:
- order the right size delivery, so you do not pay for water that overflows
- work out how many days of water you have left
- check whether a delivery has been fully pumped in
- plan how far ahead to book over summer
It also helps when you talk to a tank cleaner or water carrier, because the first thing they will ask is how big your tank is.
Common water tank sizes in New Zealand
The Waterboy’s experience is that most standard tanks in our area are 24,000, 25,000 or 30,000 litres. Some properties have two or more tanks linked together, and some have a smaller tank for garden or stock water.
Before measuring, check for a label, a moulded plate or the original paperwork. Plastic tanks often have the capacity printed near the lid or base. Concrete tanks are less likely to be marked, which is where the calculation below comes in.
Water tank size calculator: round tanks
Most household tanks are cylinders. The volume of a cylinder is:
Volume (m³) = π × r × r × h
- π is about 3.1416
- r is the radius, which is half the inside diameter, in metres
- h is the height of the water, in metres
One cubic metre holds 1,000 litres, so:
Litres = π × r × r × h × 1,000
How to measure
- Measure the inside diameter if you can. Measuring the outside includes the wall thickness and gives a figure that is too high, especially on concrete tanks with thick walls. If you can only measure the outside, the result is an upper limit.
- Halve the diameter to get the radius.
- Measure the usable height. For total capacity, measure from the floor to the overflow pipe, not to the top of the tank. Water cannot go higher than the overflow.
- Convert everything to metres before you multiply.
Worked example
A round concrete tank has an inside diameter of 3.5 metres, and the overflow is 2.6 metres above the floor.
- Radius = 3.5 ÷ 2 = 1.75 m
- Floor area = 3.1416 × 1.75 × 1.75 = 9.62 m²
- Volume = 9.62 × 2.6 = 25.0 m³
- Capacity = 25.0 × 1,000 = about 25,000 litres
That is one of the standard sizes, which is a good sign the measurements were right.
Water tank size calculator: rectangular tanks
For a square or rectangular tank, the maths is simpler:
Litres = length × width × water height × 1,000 (all in metres)
For example, a tank 2.5 m long, 2 m wide and 1.8 m to the overflow holds 2.5 × 2 × 1.8 = 9 m³, or 9,000 litres.
How to work out how much water is left
The same formula tells you how much water is in the tank right now. Instead of the overflow height, use the depth of the water.
The most useful number is litres per centimetre. On a round tank, every centimetre of depth holds the same amount:
Litres per cm = π × r × r × 10
In the worked example above, that is 3.1416 × 1.75 × 1.75 × 10 = about 96 litres for every centimetre of water.
So if the water in that tank is 80 cm deep, there are about 80 × 96 = 7,700 litres in it. Write your tank’s litres-per-centimetre figure on the lid or near the tap and checking the level becomes a one-minute job.
Remember the outlet
Your outlet is usually some way above the floor, and the water below it cannot reach your taps. To find usable water, measure the depth above the outlet, not from the floor. Level (BRANZ) notes that sediment can also build up and be drawn into a low outlet, which is another reason not to rely on the last few centimetres.
Our guide on how long a load of water lasts shows how to turn that number into days.
How much does one delivery fill?
The Waterboy delivers in two load sizes:
| Load | Price (incl GST) | Share of a standard tank |
|---|---|---|
| 6,000 litres | $235 | about a quarter |
| 10,000 litres | $345 | about two fifths |
The “share of a standard tank” figures are The Waterboy’s own guide, based on 24,000 to 25,000 litre tanks.
Using the worked example tank at 96 litres per centimetre:
- a 6,000 litre load raises the level by about 62 cm
- a 10,000 litre load raises the level by about 104 cm
Choosing the right load
Before you book, check that the space left in your tank is bigger than the load. Space left = total capacity minus the water already in it. If you have 15,000 litres of room, a 10,000 litre load fits comfortably. If you only have 7,000 litres of room, the 6,000 litre load is the right choice, and anything extra would go out the overflow.
Smaller amounts can be delivered, but because travel and the driver’s time cost the same, the full load price applies. See our pricing page for current prices and the distance charge outside the Mangawhai area.
Quick reference
| Measurement | Formula |
|---|---|
| Round tank capacity (litres) | π × r × r × h × 1,000 |
| Rectangular tank capacity (litres) | L × W × H × 1,000 |
| Litres per cm (round tank) | π × r × r × 10 |
| Space left for a delivery | Capacity minus current water |
All measurements in metres. Use the inside dimensions and the height to the overflow.
Need water or a tank clean in Mangawhai?
Once you know your tank size, ordering is easy. The Waterboy delivers Taumata Arowai certified, UV treated bore water across Mangawhai and the surrounding area. Book your water delivery online or call 0800 426 269.
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