How to use this Top-Off 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
Solve water + feed volumes so ending EC ≈ target after refill to the full mark
Why topping off is not the same as remaking
Plants remove water and nutrients at different rates. Evaporation and transpiration concentrate salts — EC climbs while the waterline drops. Adding more full-strength feed every time can push the tank hotter than your crop wants.
A top-off calculator helps you choose plain water versus nutrient mix so you land near target EC when the reservoir is full again. It is still a model; always recheck with a meter.
Rules of thumb before you trust any math
If the solution smells off, roots look slimy, or EC has been climbing for days with weird pH swings, remake the reservoir instead of nursing it with top-offs.
Use pH-adjusted water for plain top-ups. Shocking a leafy tank with alkaline tap water can lock out micronutrients overnight.
When EC falls instead of rising
Heavy feeders can pull nutrients faster than water. In that case you may need more nutrient solution and less plain water — the calculator handles both directions if you set a realistic feed EC.
Log what you added. Patterns over a week teach you your room’s typical drink rate better than any single formula.
Frequently asked questions
Should I top off with nutrient every day?
Usually no. If EC is rising, prefer plain water. If EC is falling below target, add a measured nutrient mix.
How often should I fully change the reservoir?
Many home systems remake every 7–14 days, sooner in warm rooms or after a crash. Top-offs bridge the gap; they do not replace sanitation forever.
Related calculators
- EC to PPM Converter (500 vs 700) — Convert between Electrical Conductivity (EC) and PPM across 500, 640, and 700 TDS scales.
- Nutrient Mixing Calculator — Calculate exact nutrient amounts for your reservoir based on volume and concentration ratios.
- Reservoir Volume Calculator — Calculate the volume of your reservoir in litres or gallons based on container dimensions.
Keep learning
Pair this calculator with our pH & EC guide, crop targets in plant guides, and meter picks in product reviews.