Aquarium Volume Calculator
Measure the glass once — then let the litres decide the fish, the filter and the stand.
Turn your tank's length, width and height into litres and gallons, then see the usable water after substrate, how much the filled tank weighs, a stocking guide, and the filter and heater size to aim for. Everything updates as you type.
Volume and weight are exact geometry; stocking, filter and heater figures are hobby rules of thumb — research each species and check your stand and floor.
How the volume is worked out
Centimetres in, litres out
A rectangular tank's volume is simply length × width × height in centimetres ÷ 1,000. The divide-by-1,000 isn't a fudge factor — one litre is exactly 1,000 cubic centimetres, so the conversion is precise. Measure the glass front to back for the width (also called the depth), and floor to rim for the height.
Full volume vs the water actually in there
That figure is the tank's geometric volume, and it's the number on the box. The water in it is always less: substrate takes up the bottom few centimetres, rocks and wood displace more, and you leave a gap at the top — the freeboard — so the tank doesn't slop over and the surface can break for gas exchange. A realistic working figure is about 90% of the full volume, and that's the default here. Use the water figure, not the box figure, when you dose medications or conditioners — dosing to the label size on a heavily scaped tank quietly overdoses it.
Gallons — and which gallon
US gallons = litres ÷ 3.785. Imperial gallons = litres ÷ 4.546. They're about 20% apart, so don't mix them up — and this is one place where that's a live risk rather than a pedantic footnote. The imperial gallon is the one that still turns up on British forecourts and in pub measures, but the aquarium hobby's online centre of gravity is firmly American: a "10 gallon tank" in a forum post means roughly 38 litres, not 45, and the filter and heater guides you'll read alongside it are sized in US gallons too. Retailers here price and label in litres, so the safest habit is to convert an American figure once, then think in litres from then on.
Worked example
A classic starter tank at 60 × 30 × 30 cm gives 60 × 30 × 30 = 54,000 cm³ = 54 litres, which is about 14 US gallons (or 12 imperial). At a 90% fill that's roughly 48.6 litres of water — about 48.6 kg, since water weighs 1 kg per litre. That water alone wants a filter rated around 220–320 L/h (4–6× turnover) and a heater near 50 W (about 1 W per litre). Scale it up and the weight is what surprises people: a 120 × 40 × 50 cm tank is 240 litres, which is 240 kg of water on its own, before glass, substrate or rock.
How many fish the tank can actually hold
The oldest rule of thumb in the hobby is about 1 cm of adult fish length per litre of water for a community freshwater tank. On the 48.6 litres of water in a 60 × 30 × 30 cm tank that's roughly 48 cm of adult fish — say eight 4 cm tetras with room spare, or a small school plus a bottom-dweller.
It's a planning guide and nothing more. Here's where it breaks:
- Length isn't mass. One 10 cm goldfish is nowhere near ten 1 cm tetras — body volume and waste output rise far faster than length. The rule falls apart on anything chunky.
- Adult size, not shop size. Stock to what the fish becomes. The cute 3 cm common pleco in the tank at the shop is a 40 cm adult that no starter tank can hold.
- Footprint beats volume. A tall, narrow tank holds the same litres as a long, shallow one but gives far less swimming length and surface area for gas exchange. Long and low usually stocks better.
- Species have needs the maths can't see. Schooling fish need numbers, territorial fish need space, and some species need a minimum tank length regardless of litres.
- Goldfish and marine tanks are different worlds. Both need far more water per animal than this rule suggests — don't apply it to either.
Use the number as a ceiling to stay well under, research the adult size and temperament of every species before you buy, and stock slowly — a new filter needs weeks to build the bacteria that handle the waste load.
Weight, the stand, and sizing your equipment
Water is heavy — the tank is a furniture-sized load
Water weighs 1 kg per litre. That single fact makes an aquarium one of the heaviest things in a house: a 240 L tank is 240 kg of water before you add the glass (another 40–50 kg at that size), substrate at roughly 1.5 kg per litre of gravel or sand, and any rock. Filled, that tank can top 300 kg — a quarter of a tonne and more, on a footprint the size of a sideboard.
Stand and floor
Use a solid, purpose-built stand rated for the tank — a bookshelf or a repurposed cabinet isn't it. The stand must be dead level: an out-of-level tank loads the silicone seams unevenly and is a genuine failure risk, so check with a spirit level and shim before filling. For anything above roughly 200 L, think about the floor: ideally sit the tank across floor joists rather than along a single span, keep it near a load-bearing wall, and get proper advice for upper storeys or older timber floors. And plan the position before filling — a full tank cannot be moved, only drained.
Filtration
Aim for a turnover of about 4–6× the tank volume per hour. Our 54 L example wants a filter rated around 220–320 L/h. Rated flow is measured with an empty, clean filter — real flow drops as media loads up and head height eats into it — so buying at the top of the range, or above it, is the sensible move. Heavily stocked or messy-eating tanks want more; slow-water species want less turbulence, so aim the outflow at the glass.
Heating
Budget roughly 1 watt per litre — about 50 W for the example tank, so an off-the-shelf 50 W or 75 W heater. A 240 L tank wants roughly 200–300 W, and at that size it's common to split the load across two heaters: if one sticks on or fails off, the other limits the damage. Go higher than 1 W/L if the room runs cold — an unheated room in winter, or a tank sitting well away from any heating, is better served by around 1.5 W/L.
Water changes
Plan on 20–30% of the water volume weekly for a typical community tank — about 11–16 L on our 54 L example, which is a bucket or so. Knowing the volume is what makes maintenance exact: conditioner, fertiliser and medication are all dosed per litre, so dose on the actual water volume, not the tank's glass capacity — especially in a heavily decorated tank, where the difference is the gap between the right dose and an overdose. Sizing the weekly bucket count honestly at the planning stage is worth doing, too: it's the chore that decides whether a big tank stays healthy.
❓ Frequently asked Frequently asked questions
How many litres is my aquarium?
Measure the length, width and height in centimetres, multiply them together and divide by 1,000. That works because one litre is exactly 1,000 cubic centimetres. A common starter tank at 60 × 30 × 30 cm is 54,000 cm³, which is 54 litres; a 120 × 40 × 50 cm tank comes to 240 litres. That's the tank's full geometric volume — the actual water is less once substrate, rocks and the gap you leave at the top (the freeboard) are taken out, so a realistic figure is about 90% of it, or roughly 48.6 litres in the starter-tank example.
How do I convert litres to gallons?
Divide litres by 3.785 for US gallons, or by 4.546 for imperial gallons. A 54 litre tank is about 14 US gallons or about 12 imperial gallons. The distinction matters more here than almost anywhere else: the imperial gallon is the one most people in Britain grew up with, but nearly all the aquarium hobby's online advice — and most filter and heater sizing guides — is American and means US gallons. So a "ten gallon tank" in a forum post is roughly 38 litres, not 45. Shops here sell tanks in litres, so convert an American figure once and then think in litres for dosing medications and water conditioners.
How many fish can my tank hold?
A rough starting point for a community freshwater tank is about 1 cm of adult fish length per litre of water. On the 48.6 litres of actual water in a 60 × 30 × 30 cm tank that's around 48 cm of adult fish — for example eight small tetras of about 4 cm each, with room to spare. Treat it strictly as a guide, not a rule: it ignores body mass (a 10 cm goldfish is nothing like ten 1 cm tetras), territory, schooling needs and how much waste a species produces. Research the adult size and requirements of every species before you buy, and stock slowly while the filter matures.
How much does a full fish tank weigh?
Water weighs 1 kg per litre, so a 240 litre tank holds 240 kg of water alone — before the glass, which on a tank that size can add another 40–50 kg, plus substrate at roughly 1.5 kg per litre of gravel or sand, and rocks. Filled, a tank that size can top 300 kg, and all of it lands on a small footprint. It needs a solid, level, purpose-built stand — not a bookshelf or a repurposed cabinet — and a floor that can take it. An out-of-level tank stresses the seams, and for anything above roughly 200 litres think about what's underneath: ideally sit the tank across floor joists rather than along a single span, and get advice for upper storeys.
What size filter and heater do I need?
For filtration, aim for a turnover of about 4–6 times the tank volume per hour: a 54 litre tank wants a filter rated around 220–320 litres per hour. Rated flow drops as media clogs, so buying at the top of the range is sensible. For heating, budget roughly 1 watt per litre — so about 50 W for that 54 litre tank, and around 200–300 W for a 240 litre tank, which is often split across two heaters so that one sticking on or failing off does less damage. Push higher if the room runs cold in winter or the tank sits away from any heating.
Where these figures come from
The volume, conversion and weight figures are definitions — exact by measurement, not estimates. The stocking, filter, heater and water-change figures are long-standing hobby rules of thumb, useful for planning and no substitute for researching your species.
- Volume — 1 litre ≡ 1,000 cm³, so litres = L × W × H (cm) ÷ 1,000. Exact for a rectangular tank.
- Gallons — 1 US gallon = 3.785 L; 1 imperial gallon = 4.546 L. Both are exact definitions.
- Weight — fresh water is ~1 kg per litre (1.000 kg/L at 4 °C, ~0.998 at room temperature). Substrate ~1.5 kg per litre of gravel or sand; salt water is ~2.5% heavier than fresh.
- Usable water — ~90% of geometric volume after substrate, hardscape and freeboard; adjustable at the Detailed level.
- Stocking — ~1 cm of adult fish per litre, the traditional community-freshwater guide. Not valid for goldfish, large-bodied species or marine tanks.
- Filtration & heating — turnover ~4–6× tank volume per hour; heater ~1 W per litre (~1.5 W/L in a cold room). Water changes ~20–30% weekly.
Last checked: July 2026. Volume, gallon and weight maths is exact; the stocking, equipment and water-change figures are planning guides. Research every species' adult size and requirements, use a stand rated and levelled for the tank, and check the floor loading for large setups.
Select the question that matches where you are right now.
The headline number is your tank's full volume in litres — the geometry of the glass. The breakdown turns that into gallons, the water actually in the tank once substrate and freeboard are allowed for, the filled weight, a stocking guide, and the filter and heater sizes to shop for.
Use the litres figure to shop for equipment and the water figure to dose anything. Take the weight to the stand decision before you buy the tank, not after — a filled tank can't be moved, only drained.
It's not species advice, and it's not a structural assessment. The stocking figure is a rule of thumb that ignores body mass and behaviour, and the weight figure tells you the load but not whether your floor carries it.
Tanks, medications and conditioners here are all labelled in litres. Convert an American forum figure once, then think in litres — mixing US and imperial gallons is a 20% dosing error waiting to happen.
Four things drive every number on this page: the three dimensions, how much of the tank is actually water, the stocking rule you plan to, and the sheer weight that follows from the litres.
Volume scales with all three dimensions at once, so a small measuring slip compounds. Shape matters too: the same litres in a long, low tank give more swimming length and surface area than in a tall, narrow one.
Substrate, hardscape and freeboard typically take about 10% of the box figure. A deeply scaped tank takes more — and dosing to the box size then overdoses the water that's really there.
The cm-of-fish-per-litre rule is a ceiling to stay under, not a target to hit — lower it for big-bodied or messy species, and ignore it entirely for goldfish or marine. Weight follows litres exactly at 1 kg/L, which is why the stand and the floor are a bigger constraint on tank size than the budget usually is.
A few habits keep the sums honest and the tank healthy.
Advertised tank sizes are rounded and often quote the geometric volume. Measure the glass yourself and use the water figure for dosing — it's the one that matters.
Check with a spirit level and shim as needed. An out-of-level tank loads the silicone seams unevenly, and that's a failure you only get to have once.
Rated flow drops as media clogs, so size at the top of the 4–6× range. Then add fish over weeks, not days — the filter's bacteria need time to catch up with the waste load.
A tank is a setup cost plus a running cost. Model the rest of the pet budget and the household outgoings around it.