Daily alkalinity use = how much alkalinity falls per day + anything you dosed while testing. Enter at least two readings on different days, or load them from your Companion, and the calculator gives your tank’s use in dKH a day and the daily baking soda or soda ash to replace it.
Enter your volume and at least two readings on different days.
How to get good readings
Test at the same time each day for three to five days. If you can, don’t change anything during that time: no new corals, same feeding, same dose. If you are dosing, enter the daily amount and the calculator adds it back.
Three or more readings are better than two. The calculator fits a straight line through them, so one slightly-off reading doesn’t swing the result.
From use to dose
Replacing 1 dKH a day in 100 L takes about 3.0 g of baking soda or 1.9 g of soda ash. The calculator scales this to your tank. Start a little below the figure and adjust after a few days of testing; see how often to test when you dose.
It also shows the calcium used alongside: about 7 mg/L of calcium for every 1 dKH of alkalinity used for growth. If you dose calcium separately, that’s your starting calcium dose.
Reading from your Companion
Press Use readings from my Companion. If you’re signed in, it reads your account’s alkalinity tests from the last three weeks. Otherwise, it reads the Companion saved in this browser. Nothing is sent anywhere new; it reads data you already have here.
Common questions
How much alkalinity does a reef tank use?
Anything from almost nothing in a soft-coral tank to more than 1 dKH a day in a tank full of growing stony corals.
Why is my result zero?
Your readings rose or stayed flat, so your tank isn’t using more than you add. Check your dose figure.
How often should I recalculate?
Every month or two, because use rises as corals grow.
Sources and limits
Last checked 12 October 2026. The result fits a straight line through your readings. More readings over more days give a steadier figure.
- Bulk Reef Supply: sodium bicarbonate dosing instructions
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
Keep going
Alkalinity consumption calculator · Free Reef Companion · All calculators