Reef Alkalinity Calculator
A free alkalinity calculator for reef tanks — plus calcium and magnesium. Enter your current and target levels, pick your supplement, and get the exact dose. No more guessing.
Calculate Your Dose
Actual water volume, not tank size (typically 70-80% of display volume)
Select a parameter and supplement to see dosing information.
Target Ranges at a Glance
These are widely accepted target ranges for reef aquariums. Stability within your chosen target matters more than hitting a specific number.
| Parameter | NSW | Acceptable Range | Ideal Target | Max Daily Change |
|---|---|---|---|---|
| Alkalinity | 7 dKH | 7 – 11 dKH | 8 – 9 dKH | 1 dKH |
| Calcium | 420 ppm | 380 – 450 ppm | 420 ppm | 20 ppm |
| Magnesium | 1350 ppm | 1250 – 1400 ppm | 1350 ppm | 50 ppm |
NSW = Natural Seawater. For more detail, see our Big 3 guide.
Alkalinity Units and Conversions
Alkalinity gets reported three different ways depending on where you are and which kit you use. US hobby kits almost always read in dKH. European kits and most ICP labs report meq/L. Some references still use ppm as calcium carbonate. They all describe the same thing.
| dKH | meq/L | ppm (CaCO3) | Typical read |
|---|---|---|---|
| 6.0 | 2.14 | 107 | Low — corals will show stress |
| 7.0 | 2.50 | 125 | Low end of acceptable |
| 8.0 | 2.86 | 143 | Common SPS target |
| 9.0 | 3.21 | 161 | Common mixed-reef target |
| 10.0 | 3.57 | 179 | High for SPS |
| 11.0 | 3.93 | 196 | Too high for most reefs |
| 12.0 | 4.28 | 214 | Risk of precipitation |
The math
- dKH → meq/L: multiply by 0.357
- dKH → ppm: multiply by 17.86
- meq/L → dKH: multiply by 2.8
- ppm → dKH: divide by 17.86
Safe daily change
- Alkalinity: no more than 1 dKH per day
- Calcium: no more than 20 ppm per day
- Magnesium: no more than 50 ppm per day
The calculator above splits any correction that exceeds these limits across multiple days automatically.
Why alkalinity keeps drifting down
Growing corals pull calcium and carbonate out of the water continuously to build skeleton, so a stocked reef has constant, predictable demand. Water changes alone rarely keep up once corals start growing. A slow daily decline means consumption has outpaced your dosing and you need a daily supplement schedule. A sudden drop usually means something broke — an empty two-part container, a stalled doser, or magnesium low enough to destabilize alkalinity and calcium together.
Dosing Tips
Go Slow
Never raise alkalinity more than 1 dKH/day, calcium more than 20 ppm/day, or magnesium more than 50 ppm/day. Split large corrections across multiple days.
Dissolve First
Always dissolve dry supplements in RO/DI water before adding to your tank. Never add powder directly — it creates localized pH spikes that can burn corals.
Fix Mg First
If magnesium is low, raise it before adjusting calcium or alkalinity. Low magnesium destabilizes the other two — you'll chase numbers endlessly until Mg is in range.
Test After Dosing
Wait 1–2 hours after dosing before re-testing. This allows the supplement to fully mix and react in your tank water for an accurate reading.
Spread It Out
For daily dosing, split the total into multiple smaller doses throughout the day. A dosing pump running 6–12 times daily is far better than one manual dump.
Log Everything
Track your test results and dosing amounts over time. Trends reveal consumption rate changes before they become problems. ReefTanker makes this easy.
Alkalinity Calculator FAQ
What should alkalinity be in a reef tank?
Most reef tanks run well between 8 and 9 dKH. SPS-dominant tanks are often kept slightly lower and more tightly controlled, around 7.5 to 8.5 dKH. Stability matters more than the exact number — a steady 7.8 dKH is better for corals than a tank swinging between 7 and 10.
How do I raise alkalinity in a reef tank?
Dose sodium bicarbonate (baking soda) or soda ash (sodium carbonate). Baking soda is the cheapest option and barely moves pH; soda ash raises pH along with alkalinity, which helps if your pH runs low. Always dissolve the powder in RO/DI water first, and never raise alkalinity by more than 1 dKH per day.
How do I lower alkalinity in a reef tank?
There is no supplement that safely lowers alkalinity. Stop or reduce whatever is driving it up — usually two-part dosing, a calcium reactor, or a high-alkalinity salt mix — and let your corals consume it back down. Water changes with a lower-alkalinity salt mix will speed that up. Never use acid to drop alkalinity in a display tank.
How do you convert dKH to meq/L or ppm?
Multiply dKH by 0.357 to get meq/L, or by 17.86 to get ppm as calcium carbonate. So 8 dKH is about 2.9 meq/L, or 143 ppm. Most US reef test kits report dKH, while some European kits and ICP labs report meq/L.
Why does my alkalinity keep dropping?
Growing corals consume alkalinity continuously, so a healthy tank has steady demand that water changes alone will not cover. If alkalinity falls a little every day, consumption has outgrown your dosing and you need a daily supplement schedule. A sudden drop usually points to something else — a stalled doser, an empty two-part container, or magnesium low enough to destabilize both alkalinity and calcium.
Should I fix magnesium before alkalinity?
Yes. Magnesium is what keeps calcium and alkalinity in solution, so when it is low the other two are nearly impossible to hold steady. Get magnesium into range — roughly 1,300 to 1,400 ppm — before chasing alkalinity or calcium, and raise it by no more than 50 ppm per day.
Track Your Parameters Over Time
The calculator gets you to your target. ReefTanker keeps you there — log results, spot trends, and dial in stability.