29 Gallon Aquarium chart
Geometric capacity 28.1 gallons. Rated sizes are brim-full nominal; real water volume is lower after substrate and the air gap — read the chart at your actual water line.
| Depth | Gallons | Liters | % Full |
|---|---|---|---|
| 1″ | 2 | 6 | 6% |
| 2″ | 3 | 12 | 11% |
| 3″ | 5 | 18 | 17% |
| 4″ | 6 | 24 | 22% |
| 5″ | 8 | 29 | 28% |
| 6″ | 9 | 35 | 33% |
| 7″ | 11 | 41 | 39% |
| 8″ | 12 | 47 | 44% |
| 9″ | 14 | 53 | 50% |
| 10″ | 16 | 59 | 56% |
| 11″ | 17 | 65 | 61% |
| 12″ | 19 | 71 | 67% |
| 13″ | 20 | 77 | 72% |
| 14″ | 22 | 83 | 78% |
| 15″ | 23 | 88 | 83% |
| 16″ | 25 | 94 | 89% |
| 17″ | 26 | 100 | 94% |
| 18″ | 28 | 106 | 100% |
How to measure your water volume
Measure from the inside glass bottom up to the water line — not the trim, and not from outside (glass thickness inflates the reading on small tanks). If you have substrate, measure from the top of the substrate instead and you'll get the actual water volume above it, which is the number that matters for dosing and stocking. Rated tank sizes are brim-full nominal; a typical running setup holds 10–15% less.
What one inch is worth on this tank
This tank has straight sides over its gauged range, so every inch is worth about the same — 1.56 gallons. That makes arithmetic easy and makes reading accuracy the only thing that matters: the table below is really a table of what your eyesight costs you.
| Measure | Gallons | Litres |
|---|---|---|
| One inch at the widest point (13″) | 1.60 gal | 6.1 L |
| One inch at the narrowest point (2″) | 1.50 gal | 5.7 L |
| Average inch across the full depth | 1.56 gal | 5.9 L |
| ⅛-inch reading error, worst case | 0.20 gal | 0.76 L |
| ½-inch reading error, worst case | 0.80 gal | 3.0 L |
Why your reading may not match this chart
The arithmetic on this page is exact for the geometry stated below. The disagreements you will actually see come from the tank and the measurement, not the maths — in roughly this order of size:
- Rated size is a marketing round number. Trade sizes are brim-full nominal and based on exterior dimensions. Real water volume in a running tank is commonly 10–15 % below the label once you subtract the air gap, the trim and the glass.
- Substrate displaces water. Two inches of gravel across the footprint removes roughly two inches of water volume, less the pore space the water fills — usually counted as 60–70 % of the substrate's bulk volume displaced.
- Rock and hardscape displace more than they look like. Weigh dry rock and divide by its density, or measure the water it displaces in a bucket. Estimating by eye reliably undercounts.
- Glass thickness inflates outside measurements. On a small tank, ¼-inch glass on both sides of a 12-inch width removes 4 % of the footprint. Measure inside.
What governs an aquarium's rated size
Nothing, in the sense that applies to a pressure vessel or a fuel tank. Aquarium 'gallon' ratings are trade nominal figures based on exterior dimensions, brim full. There is no listing standard behind them and no obligation on a manufacturer to hit the number. The only fixed quantity in the whole calculation is the gallon itself — exactly 231 cubic inches, per NIST Handbook 44, Appendix C.
The standard that does bite is structural. IRC Table R301.5 designs residential floors for a 40 lb/ft² live load in habitable rooms other than sleeping rooms, and 30 lb/ft² in sleeping rooms. A filled aquarium concentrates far more than that over its own small footprint — see the loading figure on the size page for this tank. Whether that matters depends on joist direction, span and where the tank sits relative to a bearing wall, which is a question for someone who can look at the framing.
Where this chart's geometry comes from
Dimensions modelled: Standard North American aquarium trade dimensions (30 x 12 x 18 in exterior-nominal). Every value in the table is computed from that geometry with a closed-form partial-fill formula — a circular or obround segment for the shell, plus scaled spherical caps where the vessel has dished heads. No interpolation, no curve fitting, no lookup tables. The derivations and the automated tolerance tests that gate each build are on the methodology page.
Nominal capacity 29 gal; geometry as modelled 28.1 gal. Where a manufacturer publishes a chart for your specific tank, that chart describes your tank and this one describes the standard geometry — prefer theirs when the two disagree and the number matters.
FAQ
How many gallons are in my 29 gallon aquarium at half depth?
About 14 gallons at 9 inches. This shell is symmetric, so half the depth is also half the volume — which is not true of every tank, and is worth checking before you assume it. Use the reader above for any specific depth.
How accurate does my stick reading need to be?
On this tank an eighth of an inch is worth up to 0.20 gallons and half an inch up to 0.8 gallons at the steepest part of the curve. Read to the nearest eighth if the number is going into a reconciliation; to the nearest half inch if you just want to know whether to order fuel.
Where exactly do I measure from?
The inside bottom of the tank, straight up to the wet line, with the stick vertical. Not from the fill-pipe rim, not from the top of a riser, and not from outside the shell. If the fill pipe is angled, the stick still has to end up vertical inside the tank or the reading runs long.
Why does my manufacturer's chart differ by a gallon or two?
Because it describes that specific tank and this one describes the stated geometry. Corner radii, internal ribs, head shape and wall thickness typically account for 1–3 %. Where the two disagree and the number matters, the manufacturer's chart for your serial number wins — and for anything that gets invoiced, neither substitutes for a calibration under ISO 7507-1.
Why is my tank's real volume lower than the rating?
Rated sizes are brim-full nominal figures taken from exterior dimensions. Subtract the glass, the air gap you actually run, and the displacement of substrate and hardscape, and a running tank commonly holds 10–15 % less than the label. Dose from the measured volume, not the label.
Sources and standards
The geometry on this page is computed, not copied. These are the documents that define the units it is expressed in and the codes that govern a aquarium in service.
- Handbook 44, Appendix C — General Tables of Units of MeasurementNational Institute of Standards and Technology, U.S. Department of Commercehttps://www.nist.gov/document/2026-nist-handbook-44-appendix-cThe legal-metrology reference for U.S. customary units. Fixes the U.S. liquid gallon at exactly 231 cubic inches and tabulates the full liquid-volume chain (minim → fluid dram → fluid ounce → gill → pint → quart → gallon).
- International Residential Code, Table R301.5 — Minimum uniformly distributed live loadsInternational Code Council (free read-only access at codes.iccsafe.org)https://codes.iccsafe.org/s/IRC2021P2/chapter-3-building-planning/IRC2021P2-Pt03-Ch03-SecR301.5Residential floors are designed to 40 lb/ft² live load in habitable rooms other than sleeping rooms, and 30 lb/ft² in sleeping rooms. A filled aquarium concentrates far more than that over its own footprint, which is why the comparison is worth doing before siting one upstairs.
- Methodology — the partial-fill formulas and liquid densities used on this sitetankvolumecalc.comhttps://tankvolumecalc.com/methodology/Every chart value here comes from a closed-form partial-fill formula, not interpolation: circular segments for cylinders, a piecewise obround model for 275/330 heating-oil tanks, and scaled spherical caps for dished heads. Densities and their bases are tabulated there.
- Special Publication 811, Appendix B.8 — Factors for Units Listed AlphabeticallyNational Institute of Standards and Technologyhttps://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8NIST's master table of conversions into SI. Factors printed in boldface are exact by definition rather than measured or rounded — which is how you tell a defined unit from an approximated one.
Every linked source above was retrieved and checked on 2026-09-02. Factors quoted as exact are exact by definition in the cited document, not rounded by us.