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Septic Tank Volume Calculator

Measure the inside of a tank and this calculator converts it to gallons, liters and cubic feet. Rectangular tanks use length by width by liquid depth; cylindrical tanks use the circular cross-section multiplied by length.

Your measurements

Results update as you type. Every unit dropdown converts for you.

Measure to the invert of the outlet, not to the top of the tank.

Tank capacity

1,197gallons

Capacity in liters
4,531 L
Volume in cubic feet
160.0 cu ft
Shape usedLength × width × liquid depth
Rectangular

Planning estimate: This calculator provides general mathematical estimates and is not a septic-system design, permit determination, soil evaluation, engineering recommendation, or substitute for local requirements. Septic regulations, design flows, loading rates, setbacks, reserve areas, and installation requirements vary by jurisdiction and site conditions.

Show the calculationOpen
  1. Liquid volume8.00 ft × 5.00 ft × 4.00 ft=160.0 cu ft
  2. Convert to gallons160.0 cu ft × 7.48052=1,197 gal

Reading your result

The gallon figure is the liquid the tank holds at the dimensions you entered — the working capacity, not the total shell volume.

Expect it to come in under the tank's nameplate rating. Ratings are nominal, wall thickness eats into inside dimensions, and freeboard above the outlet is not liquid capacity.

If you are checking an existing tank against a documented size, a result within roughly ten percent is unremarkable. A much larger gap usually means the liquid depth was measured to the lid rather than to the outlet.

How this calculator works

Volume comes from the shape of the tank interior. A rectangular tank is a box, so its liquid volume is length times width times the depth of liquid it holds. A cylindrical tank is a circle extended along its length, so its volume is π times the radius squared times the length.

For a rectangular tank, liquid depth is not the same as tank height. Liquid sits level with the outlet invert, and the space above that is freeboard — measure to the outlet, not the lid.

The cylindrical mode treats the cylinder as completely full. A partly filled horizontal cylinder needs a different calculation, so if you are gauging a part-full tank, this figure will read high.

Cubic feet convert to gallons using 7.48052 gallons per cubic foot, and gallons convert to liters using 3.785412 liters per gallon.

The formula

Rectangular volume
length × width × liquid depth
Cylindrical volume
π × radius² × length
Gallons
cubic feet × 7.48052
Liters
gallons × 3.785412

Worked example

A rectangular tank measuring 8 ft long and 5 ft wide inside, holding liquid 4 ft deep.

  1. 01Volume8 × 5 × 4=160 cu ft
  2. 02Gallons160 × 7.48052=1,197 gal
  3. 03Liters1,197 × 3.785412=4,531 L

The tank holds about 1,197 gallons of liquid — what would usually be described as a 1,200 gallon tank.

How to take the measurements

  • Measure inside dimensions. Concrete tank walls are thick, so outside measurements will overstate capacity considerably.
  • For liquid depth, measure from the tank floor to the invert of the outlet pipe rather than to the lid or the top of the wall.
  • For a cylindrical tank, measure the inside diameter through the widest point of the circle.
  • If the tank has more than one compartment, measure and calculate each compartment separately, then add them.
  • Never enter a septic tank or lean over an open tank. Take measurements from outside the opening with a rod or tape, and keep children and pets clear.

Practical considerations

  • Capacity is only half the picture. How often a tank needs pumping depends on household use and how fast solids accumulate, not on volume alone.
  • If you are comparing a measured volume against a permit record, check which figure the record quotes — nominal rating, liquid capacity and total shell volume are three different numbers.
  • Access risers and lids are often the limiting factor on getting a tank measured or serviced at all. Note their condition while you are there.
  • For a tank you cannot safely reach, a septic contractor can usually measure it during a routine pump-out at little extra cost.

Assumptions and limitations

  • The cylindrical mode assumes the cylinder is completely full. A partly filled horizontal cylinder needs a circular-segment calculation and will read high here.
  • Volume alone does not tell you whether a tank is adequately sized, correctly installed, or in serviceable condition.
  • Required tank capacity is set by your local authority and generally depends on bedrooms or design flow rather than on measured dimensions.

Frequently asked questions

Why is my calculated volume smaller than the tank's rating?

Ratings usually refer to nominal liquid capacity, while your measurement may reflect actual liquid depth and inside dimensions. Wall thickness and freeboard both account for part of the gap.

Should I measure to the lid or to the outlet?

To the outlet invert. Liquid level in a working tank sits at the outlet, and the space above it is not counted as liquid capacity.

Can I use this for a cistern or holding tank?

Yes. The geometry is identical for any rectangular or cylindrical vessel — only the labels change.

How do I handle a partly full horizontal cylinder?

This calculator assumes the cylinder is full. A partly filled horizontal cylinder needs a circular-segment calculation, which is a different formula from the one used here.

Do I measure to the lid or to the outlet?

To the outlet invert. Liquid in a working tank sits level with the outlet, and the space above it is freeboard rather than capacity.

Can I use this for a cistern or rainwater tank?

Yes. The geometry is identical for any rectangular or cylindrical vessel — only the labels change.

Why is my result smaller than the tank's rating?

Ratings are nominal and usually refer to design liquid capacity. Wall thickness reduces inside dimensions, and the space above the outlet is not counted.

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