How to use this Reservoir Volume tool
Set your options in the calculator above, read the result, then verify with a calibrated meter when the decision matters. Bookmark this page for reservoir day — consistent units beat copying numbers from charts that never name their scale or bottle rate.
Formula reference
Rectangular: L × W × H; Cylinder: π × r² × H; 1 L ≈ 0.264 US gal ≈ 0.220 UK gal
Why knowing true volume beats reading the label
Bucket and tote labels often quote brim-full capacity, but you rarely fill a hydroponic reservoir to the rim. Pumps, heaters, air stones, and float valves displace water, and you need headspace to prevent spills when returning runoff. Measuring internal dimensions and calculating usable fill height gives a volume you can trust for dosing.
Nutrient charts, pH adjuster estimates, and beneficial microbe rates all scale with litres or gallons. A 10% error in volume becomes a 10% error in EC every mix. Getting volume right once saves you from chasing phantom deficiencies caused by chronic under- or overdosing.
Common reservoir shapes in home systems
Rectangular totes and flood tables are calculated as length × width × water height. Round buckets and drums use π × radius² × height. Irregular reservoirs can be measured by filling with a known container or a water meter and recording how many litres it took to reach your operating line.
For tapered pots or nested trays, average the top and bottom cross-sections or use a water-fill method. Mark a permanent fill line on the outside once you know the working volume so future remakes stay consistent without remeasuring every time.
Litres versus US and UK gallons
One litre is 0.264172 US gallons and about 0.219969 UK (imperial) gallons. US nutrient labels usually mean US gallons; older UK gardening references may mean imperial. Mixing those up on a large reservoir can swing your dose by nearly 20%.
When sharing recipes online, state both the volume unit and the nutrient rate unit. “2 ml per litre in a 20 L res” travels better than “a tablespoon per gallon” without specifying which gallon. Convert once, write it on the reservoir lid, and stick to that system.
Sizing a reservoir for your system type
Deep water culture and recirculating DWC need enough volume to buffer temperature and nutrient swings — often several litres per plant site. Nutrient film technique and drip systems can run smaller reservoirs but deplete faster, so top-up frequency rises.
As a rough planning guide, many hobby leafy-green systems aim for at least 3–5 litres of solution per mature plant in recirculating setups, with more volume preferred in warm rooms. Larger reservoirs change chemistry more slowly, which is forgiving for beginners but takes longer to correct if you overdose.
Frequently asked questions
Should I include the volume displaced by pots and media?
For dosing, use the actual liquid volume at your fill line. If net pots and clay pebbles displace water, measure volume with those in place, or fill to the line and note how much water you added.
How do I measure a reservoir that is already installed?
Mark the intended water line, empty or lower the level, then refill using a measured jug or flow meter until you hit the mark. That measured fill is your working volume.
Does temperature change the volume enough to matter?
Thermal expansion of water is tiny across normal grow-room ranges. Temperature matters far more for dissolved oxygen and root health than for litre calculations used in dosing.
Related calculators
- Hydroponic System Selector — Answer a few questions to get a personalised hydroponic system recommendation for your setup.
- Nutrient Mixing Calculator — Calculate exact nutrient amounts for your reservoir based on volume and concentration ratios.
- pH Adjustment Calculator — Estimate how much pH Up or pH Down solution is needed to reach your target pH.
Keep learning
Pair this calculator with our pH & EC guide, crop targets in plant guides, and meter picks in product reviews.