Reservoir Volume & Top-Off Calculator

Reservoir Volume & Top-Off Calculator

Size your reservoir in gallons and liters from tank dimensions, then plan top-offs — see exactly how adding water changes your EC and PPM, or how much plain water it takes to hit a target EC. Need help choosing a tank? Talk to a Grow Expert.

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Reservoir Volume & Top-Off Calculator

Choose a tool: Reservoir Volume to size a tank from its dimensions, or Top-Off & Dilution to manage EC when adding water.

Tip: Measure inside dimensions of the tank for the most accurate capacity. Most growers fill to 85–90% to leave headspace for pumps, air stones, and splash.
How to use this calculator
  1. Pick a tool. Use Reservoir Volume to size a tank, or Top-Off & Dilution to manage EC when adding water.
  2. Volume: choose the tank shape, enter inside dimensions, and set your fill level — most growers run 85–90%.
  3. Top-Off (Blend Result): enter current volume, current EC, and how much water you're adding to see the resulting EC and PPM.
  4. Top-Off (Hit Target EC): enter your target EC and the calculator tells you exactly how many gallons of top-off water to add.
  5. Copy or share your results — the link restores your exact inputs.

Formulas: Rectangular gallons = L × W × H (in) ÷ 231  ·  Round gallons = π × r² × H (in) ÷ 231  ·  Blend EC = (V₁E₁ + V₂E₂) ÷ (V₁ + V₂)  ·  Dilution volume = V₁ × (E₁ − Etarget) ÷ (Etarget − Ewater)

Common Reservoir Sizes — Dimensions to Gallons Quick Reference
Shape Inside Dimensions (in) Full Capacity At 90% Fill
Round bucket 11.9 dia × 14.5 ≈ 7.0 gal ≈ 6.3 gal
Rectangular tote 24 × 16 × 16 ≈ 26.6 gal ≈ 23.9 gal
Round barrel 22 dia × 24 ≈ 39.5 gal ≈ 35.5 gal
Rectangular tank 36 × 24 × 18 ≈ 67.3 gal ≈ 60.6 gal
Rectangular tank 48 × 24 × 20 ≈ 99.7 gal ≈ 89.7 gal
Round drum 30 dia × 36 ≈ 110.2 gal ≈ 99.2 gal

Frequently Asked Questions

How do I calculate reservoir volume in gallons?
For rectangular tanks, multiply length × width × depth in inches, then divide by 231 (the number of cubic inches in a US gallon). For round barrels, use π × radius² × height ÷ 231. Always measure inside dimensions — thick-walled totes can overestimate by a gallon or more if you measure outside the walls.

How full should I fill my hydroponic reservoir?
Most growers run 85–90% of full capacity. The headspace leaves room for air stones and return lines, prevents splash-over from aeration, and gives you a buffer when adding nutrients or pH adjusters. In recirculating systems, also account for the solution held in lines and grow sites when the pump is running.

Does topping off with plain water lower my EC?
Yes. Adding RO or low-EC water dilutes the solution, so EC drops proportionally to the volume added. Use the Blend Result mode to see the exact new EC, or Hit Target EC to work backwards from the EC you want. If plants are drinking water faster than nutrients, EC can actually rise between top-offs — that's normal, and plain-water top-offs bring it back down.

Should I top off my reservoir or do a full change?
Top off daily or as needed to replace what plants drink, but still do a full reservoir change every 7–14 days. Top-offs keep volume and EC stable in the short term, but nutrient ratios drift over time as plants take up elements at different rates — a full change with fresh solution resets the balance.

How do I convert EC to PPM?
Multiply EC (mS/cm) by 500 for the US/Hanna scale, or by 700 for the Truncheon/Australian scale. An EC of 2.0 reads as 1000 ppm on a 500-scale meter and 1400 ppm on a 700-scale meter. Always confirm which scale your meter uses before comparing numbers to a feed chart.
🛢️ Reservoir Fill Visual — See Your Working Volume Visual ▾
Working volume (filled)
Headspace
Fill line

The tank redraws live as you change dimensions and fill level in the Reservoir Volume tab. Headspace above the fill line leaves room for air stones, return lines, and nutrient additions.

Found your reservoir size?

Shop round, rectangular, and tote-style reservoirs from 5 to 550 gallons — lids, fittings, and float valves included.
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