How Many Gallons Is My Fish Tank?
Knowing the precise water volume of your aquarium is the foundation of good fishkeeping. It determines how many fish you can safely keep, how much dechlorinator and medication to dose, what filter turnover rate you need, and how much water to change each week. Our aquarium tank volume calculator handles seven common tank shapes — including cylinder, sphere, hexagon, corner, and L-shape — with support for glass thickness deductions and usable water estimates. All results display in both gallons and litres, with every measurement shown in both inches and centimetres.
How to Use the Tank Volume Calculator
The calculator updates instantly as you type — no submit button needed. Here is a step-by-step walkthrough:
Select your tank shape
Choose from rectangular, bow front, cylinder, sphere, hexagon, corner pentagon, or L-shape. The input fields update automatically to match the dimensions each shape requires.
Choose your input unit
Toggle between Inches and Centimetres. Results always display in US gallons, UK gallons and litres — all three units shown simultaneously regardless of which input unit you choose.
Enter your dimensions
Measure the outside of your tank with a tape measure. If you only have inner dimensions, set glass thickness to "Not included — measuring inside" to avoid double-subtracting the panels.
Set glass thickness
Select from presets ranging from 5 mm (nano tanks) to 19 mm (very large). The calculator subtracts this from length, width, and height (×2 per axis) to give the true interior volume. Thicker glass on larger tanks can reduce volume by several gallons.
Adjust deduction sliders
Set your substrate depth, headspace at the top, and percentages for decorations and equipment. The usable water figure accounts for all of these — use it for medication dosing and bioload calculations.
Read your results
The results panel shows gross volume (gal + L), usable water, filled tank weight (lbs + kg), and your 25% weekly water change amount. Every dimension readout shows both inches and centimetres simultaneously.
How Tank Volume Is Calculated for Each Shape
Every shape has a different internal geometry, so the volume calculation works differently depending on what you're measuring. The calculator handles all of this automatically — just enter your dimensions and it converts instantly to US gallons, UK gallons and litres.
Rectangular tank
The simplest shape. The calculator multiplies length × width × height to get the total interior space, then converts to litres and gallons. Glass thickness is subtracted from all three dimensions before multiplying — a 10 mm panel on each side reduces the effective length, width and height accordingly.
Bow front tank
The curved front glass bulges outward, so the true internal volume is less than a flat-fronted tank of the same outer dimensions. The calculator applies a standard correction for the curved panel — the result is roughly 87% of what a flat-fronted rectangle of the same measurements would hold. Measure the maximum width at the bow's deepest point.
Cylinder tank
A circular cross-section holds less than a square of the same width. The calculator uses the diameter and height to find the volume of the circular column of water. Enter the outer diameter — glass thickness is subtracted before calculating the interior space.
Sphere / fish bowl
A full sphere is calculated from the diameter alone. In practice, fish bowls are never filled to 100% of their sphere volume — the headspace and substrate sliders let you set the actual fill level, and the usable water figure reflects this. Enter the outer diameter across the widest point.
Regular hexagon tank
A six-sided tank is measured by the length of one flat side panel — all six sides are equal. The calculator works out the total footprint from this measurement, then multiplies by height. A hexagon holds about 65% of the volume of a rectangle drawn around it.
L-shape tank
An L-shaped tank is treated as two separate rectangular sections joined together. Measure each arm's length and width independently, making sure the measurements don't overlap at the join. The calculator adds both arm volumes and applies the shared height. Use outside dimensions and set glass thickness as normal.
Always measure the outside of your tank unless you can reach inside easily. Then use the glass thickness dropdown to subtract panel thickness from all dimensions. Measuring inside a filled tank is impractical — outside measurements plus glass deduction is the standard approach.
Aquarium Tank Volume Calculator
Enter your tank dimensions below. All results display in US gallons, UK gallons and litres — every measurement shown in all three volume units and both inches and centimetres simultaneously.
Standard Aquarium Sizes — Dimensions & Volume
Aquarium manufacturers use nominal sizes (10 gallon, 55 gallon, etc.) that are approximate. Actual volume varies slightly between brands, and the usable water volume is always less than the nominal rating once substrate, headspace, and decorations are accounted for.
| Nominal size | Exterior L × W × H | Gross volume | Usable water est. | Full weight est. | Best use |
|---|---|---|---|---|---|
| 2.5 gallon | 12 × 6 × 8"30 × 15 × 20 cm | 2.5 US gal2.1 UK gal · 9.5 L | ~1.8 / ~1.5 / ~7US gal / UK gal / L | ~27 lbs / ~12 kg | Shrimp / betta |
| 5 gallon | 16 × 8 × 10"41 × 20 × 25 cm | 5 US gal4.2 UK gal · 18.9 L | ~3.8 / ~3.2 / ~14US gal / UK gal / L | ~48 lbs / ~22 kg | Nano community |
| 10 gallon | 20 × 10 × 12"51 × 25 × 30 cm | 10 US gal8.3 UK gal · 37.9 L | ~7.8 / ~6.5 / ~30US gal / UK gal / L | ~94 lbs / ~43 kg | Beginner community |
| 20 gallon long | 30 × 12 × 12"76 × 30 × 30 cm | 20 US gal16.7 UK gal · 75.7 L | ~15.8 / ~13.2 / ~60US gal / UK gal / L | ~183 lbs / ~83 kg | Planted / community |
| 20 gallon high | 24 × 12 × 16"61 × 30 × 41 cm | 20 US gal16.7 UK gal · 75.7 L | ~15.4 / ~12.8 / ~58US gal / UK gal / L | ~181 lbs / ~82 kg | Display / tall plants |
| 29 gallon | 30 × 12 × 18"76 × 30 × 46 cm | 29 US gal24.2 UK gal · 109.8 L | ~22.8 / ~19.0 / ~86US gal / UK gal / L | ~260 lbs / ~118 kg | Community |
| 40 breeder | 36 × 18 × 16"91 × 46 × 41 cm | 40 US gal33.3 UK gal · 151.4 L | ~31.5 / ~26.2 / ~119US gal / UK gal / L | ~356 lbs / ~161 kg | Breeding / cichlids |
| 55 gallon | 48 × 13 × 21"122 × 33 × 53 cm | 55 US gal45.8 UK gal · 208.2 L | ~43.5 / ~36.2 / ~165US gal / UK gal / L | ~497 lbs / ~225 kg | Full community |
| 75 gallon | 48 × 18 × 21"122 × 46 × 53 cm | 75 US gal62.5 UK gal · 283.9 L | ~60.5 / ~50.4 / ~229US gal / UK gal / L | ~665 lbs / ~302 kg | Large community |
| 90 gallon | 48 × 18 × 24"122 × 46 × 61 cm | 90 US gal75.0 UK gal · 340.7 L | ~72.8 / ~60.6 / ~276US gal / UK gal / L | ~788 lbs / ~357 kg | Cichlids / reef start |
| 125 gallon | 72 × 18 × 22"183 × 46 × 56 cm | 125 US gal104.1 UK gal · 473.2 L | ~101 / ~84 / ~382US gal / UK gal / L | ~1,085 lbs / ~492 kg | Large display |
| 180 gallon | 72 × 24 × 25"183 × 61 × 64 cm | 180 US gal150.0 UK gal · 681.4 L | ~147 / ~122 / ~556US gal / UK gal / L | ~1,540 lbs / ~699 kg | Reinforce floor |
Tanks over 75 gallons (284 L) typically weigh more than 600 lbs (272 kg) when full. Always verify your floor's load-bearing capacity before filling a large aquarium — residential floors typically support 40–50 lbs per square foot. Consult a structural engineer for tanks over 100 gallons if your floor gives you any concern.
Aquarium Glass Thickness Guide
Glass thickness is selected by manufacturers based on the hydrostatic pressure the panel must withstand at a given height. Using glass that is too thin for a tall or large tank risks catastrophic failure. The table below shows standard glass thickness for common tank sizes.
| Glass thickness | Typical use | Max recommended height | Volume range |
|---|---|---|---|
| 5 mm (3/16") | Nano tanks, shallow displays | ~12" / 30 cm | 2–10 gallons / up to ~38 L |
| 6 mm (1/4") | Standard small tanks | ~18" / 46 cm | 10–29 gallons / 38–110 L |
| 8 mm (5/16") | Medium-small tanks | ~20" / 51 cm | 20–40 gallons / 76–151 L |
| 10 mm (3/8") | Standard medium tanks | ~22" / 56 cm | 30–75 gallons / 114–284 L |
| 12 mm (15/32") | Large tanks | ~24" / 61 cm | 75–125 gallons / 284–473 L |
| 16 mm (5/8") | Extra-large tanks | ~28" / 71 cm | 125–200 gallons / 473–757 L |
| 19 mm (3/4") | Very large / commercial | ~36" / 91 cm | 200+ gallons / 757+ L |
Acrylic tanks of the same volume can use thinner panels than glass — typically 0.4× the glass equivalent — but scratch far more easily. Many acrylic tanks also have slight optical distortion. For volume calculation purposes, measure and treat acrylic thickness the same way as glass.
How Tank Shape Affects Fishkeeping
Two tanks with the same nominal volume can have dramatically different surface area, swimming space, and suitability for different fish. Shape is one of the most under-considered factors when choosing an aquarium.
Rectangular (long)
The gold standard for fishkeeping. Maximises surface area for gas exchange and provides the most usable horizontal swimming space. The 40 breeder (36×18×16") is particularly popular for its wide, shallow footprint — ideal for bottom-dwellers, corydoras shoals, and planted tanks.
Rectangular (tall / high)
Same footprint as long tanks but more height. Looks dramatic and suits tall stem plants, discus, and angelfish. Less surface area per volume means lower oxygen capacity. Requires a more powerful filter to circulate water through the full water column.
Bow front
Curved front glass gives a wider viewing angle and a slightly larger interior depth. Volume is approximately 87% of an equivalent rectangle. Popular for living room display tanks. The curved front does make attaching external equipment slightly more challenging.
Cylinder
Visually stunning but less practical. Surface area is significantly lower than a rectangle of the same volume. Filtration is tricky — most HOB filters cannot attach to a curved rim. Best suited for nano displays or jellyfish tanks with circular flow systems. Volume ~78% of bounding rectangle.
Hexagon
A classic centrepiece shape. Provides a 360° viewing angle when positioned in the centre of a room. Volume is roughly 65% of the bounding rectangle. Surface area to volume ratio is lower than long tanks. Better suited to display purposes than high-density community stocking.
Corner & L-shape
Space-efficient designs that tuck into room corners or follow wall layouts. Both shapes provide good surface area. Volume calculations are more complex — use the calculator rather than estimating. L-shapes work well as dividing features in open-plan rooms.
How Much Does a Full Aquarium Weigh?
Water is extremely heavy — 8.34 lbs per US gallon (1 kg per litre). Most people significantly underestimate how much a filled aquarium weighs. The table below gives a quick sense of what to expect.
| Tank size | Water weight | Glass + stand est. | Total est. | Floor note |
|---|---|---|---|---|
| 10 US gal / 8.3 UK gal / 38 L | ~83 lbs / ~38 kg | ~12 lbs / ~5 kg | ~95 lbs / ~43 kg | Standard floor |
| 20 US gal / 16.7 UK gal / 76 L | ~167 lbs / ~76 kg | ~24 lbs / ~11 kg | ~191 lbs / ~87 kg | Standard floor |
| 40 US gal / 33.3 UK gal / 151 L | ~334 lbs / ~151 kg | ~50 lbs / ~23 kg | ~384 lbs / ~174 kg | Standard floor |
| 55 US gal / 45.8 UK gal / 208 L | ~459 lbs / ~208 kg | ~70 lbs / ~32 kg | ~529 lbs / ~240 kg | Standard floor |
| 75 US gal / 62.5 UK gal / 284 L | ~626 lbs / ~284 kg | ~100 lbs / ~45 kg | ~726 lbs / ~329 kg | Check if joists span >12' |
| 125 US gal / 104.1 UK gal / 473 L | ~1,043 lbs / ~473 kg | ~150 lbs / ~68 kg | ~1,193 lbs / ~541 kg | Verify floor capacity |
| 180 US gal / 150.0 UK gal / 681 L | ~1,501 lbs / ~681 kg | ~220 lbs / ~100 kg | ~1,721 lbs / ~781 kg | Engineer assessment advised |
Position large aquariums perpendicular to floor joists where possible so the weight distributes across multiple joists. Running a tank parallel to a single joist concentrates all the load on one structural member. For tanks over 75 gallons on an upper floor, consult a structural engineer before installation.
Why Accurate Tank Volume Is Essential
Medication dosing
Most aquarium medications are dosed per gallon or per 10 gallons (or per 10 litres). An error of 20% in your volume estimate can result in significant under- or overdosing. Underdosing is ineffective; overdosing can be fatal to fish and invertebrates, particularly shrimp and scaleless species.
Water conditioner & dechlorinator
Dechlorinators like Prime are dosed by volume. During water changes you only treat the new water being added — knowing exactly how many gallons (or litres) you remove and replace lets you dose precisely and avoid wasting expensive conditioner.
Stocking limits
Bioload-based stocking calculations depend entirely on accurate water volume. Overestimating your tank volume by even 10 gallons (38 L) can result in significantly overstocking — with real consequences for water quality and fish health. Use the stocking calculator with the volume figure from this tool.
Filter and pump sizing
Filters are rated by flow rate (gallons per hour / litres per hour). The common guideline is to turn over tank volume 4–10× per hour. A 55-gallon (208 L) tank needs a filter rated for 220–550 GPH (833–2,082 LPH). Using gross vs. usable volume can shift this recommendation considerably.
Frequently Asked Questions
Answers to the most common aquarium volume questions.
How do I calculate the volume of a fish tank in gallons?
How many gallons is a 48×13×21 tank?
Does glass thickness affect the volume of an aquarium?
How much does a full fish tank weigh?
What is the difference between gross volume and usable water volume?
How do I convert fish tank gallons to litres?
How do I measure the volume of a hexagonal or cylinder fish tank?
Why does tank shape matter for fishkeeping?
What is the right tank size for a beginner?
How much water should I change each week?
Plan Your Aquarium with Our Stocking Calculator
Once you know the exact volume of your tank, the next step is working out how many fish it can safely support. Our Aquarium Stocking Calculator uses a bioload-based method covering 100+ freshwater, saltwater, and brackish species — with live compatibility warnings, school-size alerts, and filtration quality adjustments.
Use the volume figure from this tool as your tank size input in the stocking calculator for the most accurate results.