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Construction & HomeplumbingUnited States only

Water Tank Capacity Calculator

Water tank capacity and how long it lasts. A horizontal cylindrical tank does not fill linearly, so a dipstick reading half depth means considerably less than half full at the ends and exactly half in the middle.

Also called: tank capacity calculator, water storage calculator.

Tank shape
m
m
m
m
litres/day
Water in the tank
900

900 litres at a 0.6 m depth, of a 1,500 litre capacity, so the tank is 60% full. At 540 litres a day that lasts 1.67 days. A rectangular tank fills linearly, so depth and volume are proportional.

Full capacity
1,500
Percentage full
60
Household daily demand
540
Days of supply left
1.67
Days of supply when full
2.64
Usable capacity
1,425
On the shape
A rectangular tank fills linearly, so depth and volume are proportional.
How this is calculated
Method and background

How this is calculated

Rectangular and vertical cylindrical tanks fill linearly, so depth is proportional to volume. A horizontal cylinder does not: the cross section is a circular segment, widest at the middle, so the first quarter of depth holds much less than the second. That is why a horizontal tank needs a calibrated dipstick rather than an evenly marked one. The days of supply figure uses the household demand you enter, and a usable capacity of ninety-five percent is applied because the outlet sits above the floor.

a horizontal cylinder does not fill linearly, so depth and volume need the circular segment area
theta
Half-angle subtended by the water surface

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

a rectangular tank at 60 percent depth

Tank shape
Rectangular
Length or diameter
1.5 m
Width
1 m
Height
1 m
Current water depth
0.6 m
People in the household
4
Water use per person
135 litres/day

Water in the tank900

1.5 x 1 x 0.6 x 1000; 4 x 135

Open this example

a horizontal cylinder is half full at half depth

Tank shape
Horizontal cylinder
Length or diameter
1.5 m
Width
1 m
Height
2 m
Current water depth
0.75 m
People in the household
4
Water use per person
135 litres/day

Water in the tank1,767.1

boundary: the one depth where the non-linear tank agrees with a linear one

Open this example

Method and limits

What it assumes

  • A usable capacity of 95 percent, since the outlet is above the tank floor.

What it deliberately does not model

  • Sediment reduces usable capacity over time.
  • Water use per person varies enormously with garden watering and appliance efficiency.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Why is my horizontal tank not half full at half depth?
It is, exactly at the midpoint. Away from the midpoint the relationship is not linear, because the cross section is a circular segment that narrows towards top and bottom.
Can I use the full capacity?
Not quite. The outlet sits above the floor, so the last few percent cannot be drawn. This applies a 95 percent usable figure.