Cubic Meters to Gallons

Your Pool Is in Cubic Metres, the Chemical Label Is in Gallons

A pool built to metric drawings gives up its volume in cubic metres: length times width times average depth, straight off the tape measure. Then the chlorine, the algaecide and the stabiliser arrive with dosing instructions written per 10,000 US gallons, the unit the pool-chemical trade standardised on. Getting from one to the other is the difference between a correct dose and a guess.

Conversion factor: 1 m³ = 1,000 L = 264.172052 US gallons. An 8 × 4 m pool with an average depth of 1.4 m holds 44.8 m³, which is 11,835 US gallons — so anything the label says to add per 10,000 gallons has to be multiplied by about 1.18.

Getting the Volume Right First

Rectangular pools are the simple case

Length × width × average depth in metres gives cubic metres directly. A 10 × 5 m pool averaging 1.5 m deep is 75 m³, and no further arithmetic is needed before converting.

Round and oval shells

For a circular above-ground pool use π × radius² × depth. A 4.5 m diameter shell filled to 1.2 m works out at about 19.1 m³. An oval is close enough to length × width × depth × 0.79.

Sloping floors need an average

Take the shallow-end depth plus the deep-end depth and halve it. Using the deep end alone can overstate a pool's volume by a third, which means overdosing every single treatment.

Spas sit on another scale entirely

A 2 × 2 m hot tub filled to 0.85 m is only 3.4 m³, roughly 898 gallons — under a tenth of a family pool, so a pool-sized scoop would be wildly wrong.

Working a Dose Out from a Metric Volume

Measure once, convert once, and keep the gallon figure written on the pump-house wall — you will use it every time you treat the water.

Multiply your measurements into cubic metres

Length, width and average depth in metres, multiplied together, give the volume in m³ without any unit juggling. For a round shell, square the radius, multiply by π and then by the depth.

Type the cubic metres and read the gallons

Enter 44.8, 75, 115.2 — whatever you calculated. The gallon figure appears as you type, and a comma is accepted as the decimal point if that is how you write it.

Scale the label dose to your own volume

Divide your gallon figure by 10,000 and multiply the stated dose by the result. At 11,835 gallons that is a factor of 1.18, so a label calling for one pound of shock becomes about 1.18 pounds.

Reverse it when the figure comes in gallons

Builders' brochures and US equipment sheets quote gallons. Press the swap button (↔) to run gal → m³, or switch the right dropdown to litres if your local supplier doses per cubic metre instead.

Dose the water you actually have: a pool run a few centimetres below the tile line, or one with a large attached spa plumbed into the same circuit, is not the volume on the original drawing. Re-measure the waterline depth rather than trusting the design figure.

Pool Sizes in Cubic Metres and Gallons

Typical domestic pools and spas, with the average depth used for each, so you can find a shape close to yours and sanity-check your own measurement before you dose anything.

Pool shape and size	Average depth	Volume (m³)	Volume (US gal)
Hot tub, 2 × 2 m	0.85 m	3.4	898
Round above-ground, 4.5 m across	1.2 m	19.1	5,042
Plunge pool, 6 × 3 m	1.2 m	21.6	5,706
Round above-ground, 5.5 m across	1.3 m	30.9	8,159
Small family pool, 8 × 4 m	1.4 m	44.8	11,835
Standard rectangle, 10 × 5 m	1.5 m	75.0	19,813
Lap pool, 25 × 3 m	1.4 m	105.0	27,738
Large rectangle, 12 × 6 m	1.6 m	115.2	30,433

Notice how quickly the gallon column climbs: an 8 × 4 m pool and a 12 × 6 m one look like neighbours on a plan, yet the larger holds well over twice the water and needs well over twice the chemical. Remember too that steps, benches and a beach entry take a few cubic metres out of the theoretical rectangle.

Points Worth Knowing at the Poolside

Metres in, gallons out as you type

Both fields track each other live, so you can run through two or three candidate average depths and watch the gallon figure move before settling on one.

Litres too, for a European label

Searchable dropdowns on both sides carry the full volume list, so the same measurement also gives litres when the product in your hand doses per cubic metre.

A builder's gallon figure back into metres

The swap button reverses the direction, which is what you want when the only volume you have came off a US brochure or an imported pump specification.

A clean number for the maintenance log

Copy hands over the value on its own with no unit attached, ready to paste into a water-testing app, a spreadsheet or a note pinned by the filter.

Pool Volume and Dosing Questions

How do I work out my pool's volume from its dimensions?

Find the water's surface area, then multiply by the average depth. A rectangle is length × width × average depth. A circle is π × radius² × depth, so a 4.5 m shell at 1.2 m is 3.1416 × 2.25² × 1.2 ≈ 19.1 m³. An oval is close enough to length × width × depth × 0.79. Work in metres and the answer is already in cubic metres.

Why do pool chemical labels dose per 10,000 gallons?

It is a convenient round batch close to the size of an average North American residential pool, and the industry standardised on it so dosing tables stay simple: one figure to raise chlorine by a given number of parts per million, another for pH, another for stabiliser. It is a reference quantity, not a pool size — scale it by dividing your own gallon figure by 10,000.

What average depth should I use for a pool with a sloping floor?

For a constant slope, add the shallow-end and deep-end depths and divide by two — 1.0 m and 1.8 m gives 1.4 m. Pools with a flat shallow shelf, a sharp hopper and then a flat deep floor are better split into sections and added together. The error matters: a 10 × 5 m pool is 50 m³ at 1.0 m average but 90 m³ at 1.8 m.

Can I use the same dose scale for a hot tub as for the pool?

Only if you scale it down properly, and preferably not at all. A 3.4 m³ spa is roughly 898 gallons — under a tenth of the 10,000-gallon reference — so a pool measure is a tenfold overdose. Hot water also burns through sanitiser far faster and holds it less well, so spa products are formulated differently. Use a spa product against the spa's own gallon figure.

How does the volume figure set my filter's flow rate and turnover?

Turnover is the time the pump needs to push the whole volume through the filter once, and residential practice usually aims for six to eight hours. Divide the volume by the target hours: 75 m³ in eight hours is 9,375 L per hour, or about 41 US gallons per minute. A pump quoted in gallons per minute and a pool measured in cubic metres cannot be compared until one is converted.