CO₂ cylinder runtime estimator
Weigh your cylinder, enter your bubble rate and photoperiod, and find out exactly when you'll run out — with a reorder reminder built in so you're never caught short.
How to measure your remaining CO₂
The most reliable way to know how much CO₂ is left in your cylinder is to weigh it. Place the cylinder on kitchen scales — even a basic postal scale accurate to 1–2 grams is sufficient for this purpose. Then subtract the tare weight: the mass of the empty cylinder itself.
The tare weight is stamped or engraved on the cylinder valve body, typically expressed as "TW" followed by a number in grams or kilograms. On paintball cylinders (88 g) it is usually cast into the aluminium body. On larger cylinders from gas suppliers, it is stamped on the brass collar around the valve. The difference between your current scale reading and the tare weight is your remaining CO₂ in grams.
Pressure gauges on regulators are unreliable for estimating remaining CO₂. CO₂ cylinders contain liquefied gas — pressure remains roughly constant until almost all the liquid has converted to gas, at which point pressure drops sharply. This means a gauge can show adequate pressure right up until the cylinder is nearly empty, then drop suddenly over a day or two. Weighing eliminates this problem entirely.
Tare weight location
On paintball cylinders, look for "TW" on the brass valve head. On larger fire-extinguisher-style cylinders, check the metal collar around the valve or the cylinder body near the top. On aluminium cylinders sold in the UK for aquarium use, it is typically engraved on the side of the valve assembly.
Diffuser type changes everything
Not all CO₂ you pump into your tank reaches the water as dissolved gas. The diffuser type determines dissolution efficiency — the percentage of injected CO₂ that actually dissolves rather than escaping as bubbles through the surface.
An inline diffuser fitted in the return line of an external filter is the most efficient method. CO₂ is injected into the pressurised water flow and given extended contact time with the water in the filter hose before re-entering the tank. Dissolution efficiency is 90–95%. A quality glass or ceramic in-tank diffuser producing fine micro-bubbles achieves 80–90% efficiency. An airstone bubbling large CO₂ bubbles through the water achieves 60–70%, because large bubbles have a lower surface-area-to-volume ratio and rise to the surface before fully dissolving.
The practical consequence is substantial. At 1 bubble per second (bps) over a 10-hour photoperiod, switching from an airstone (70%) to an inline diffuser (95%) cuts CO₂ consumption by about 26% while delivering the same dissolved CO₂. Over the lifetime of a cylinder this can add weeks of runtime.
Why the 7-day reorder buffer matters
CO₂ regulators are notoriously unreliable as empty-cylinder indicators. As described above, pressure stays roughly constant until liquid CO₂ is exhausted, then drops sharply. When a cylinder runs out overnight, plants enter the next morning's photoperiod with no CO₂ — causing algae spikes in high-light planted tanks within a day or two of disrupted CO₂ supply.
The 7-day reorder date in the calculator is set earlier than the estimated run-out date. Order a replacement cylinder or schedule a refill by the reorder date, and you will always have CO₂ in hand before you run out. Many aquarium CO₂ suppliers in the UK offer next-day delivery on CO₂ cylinders, so a 7-day buffer provides ample time even accounting for delivery delays.
For tanks where CO₂ stability is critical — particularly high-tech planted tanks with carpet plants susceptible to CO₂ fluctuation — having a spare 88 g paintball cylinder as emergency backup is a worthwhile precaution. The drop checker colour guide helps you monitor CO₂ levels between sessions, and the AquaCalc pH Monitor gives real-time CO₂ readings throughout the photoperiod.