Salt cost per litre = pack price ÷ pack weight in kg × your mixing ratio in grams per litre ÷ 1,000. That tells you what the salt you actually use costs, not just the price on the bucket. Add the cost of water separately, and use the ratio for your salt at your target salinity rather than assuming every salt goes as far.
Four things to note before comparing buckets
Write down the exact salt, the pack weight, the price including delivery and the mixing ratio for your target salinity. The label or the maker’s instructions are a better starting point than a ratio borrowed from another brand. If you’ve worked out your own ratio, note how you got it.
The planner records your target salinity in ppt, but doesn’t turn that into a weight of salt. The ratio is a separate input. For example, aiming for 35 ppt doesn’t mean you need 35 grams of a particular salt mix per litre.
A simple salt price example
Just to show the maths, take a 20 kg pack costing £60 and a made-up ratio of 35 g/L. The ratio is invented for this example and isn’t a mixing instruction for any real salt.
| Step | Calculation | Result |
|---|---|---|
| Price per kg | £60 ÷ 20 | £3/kg |
| Salt per litre | 35 ÷ 1,000 | 0.035 kg |
| Salt cost per litre | £3 × 0.035 | £0.105 |
| Salt for a 20 L batch | 20 × 0.035 | 0.7 kg |
| Salt cost per batch | 0.7 × £3 | £2.10 |
From one batch to a yearly budget
A 200 L system with a 10% change needs a 20 L batch. Changing every seven days, the budget works on 365 ÷ 7 = 52.142857 batches a year. With the example figures, that’s 36.5 kg of salt costing £109.50 a year. This is a budget, not advice on how often to change water.
If you enter water at £0.10/L, each batch adds £2 of water, or £104.29 a year after rounding. Salt and water together come to £213.79 a year. The monthly figure is an average; in reality you buy packs at particular times.
A 0% change means no batches and no salt. The tool won’t claim one bucket lasts forever in that case. Planning dates on a calendar is a separate job from working out a yearly cost.
Compare how far it goes, not just the price
A pack that’s cheaper per kilogram can cost more per litre of mixed water if you need more of it. Run both salts at the same target salinity and with comparable delivered prices. Ease of handling, suitability for your tank and the maker’s instructions matter too, not just pennies.
Red Sea’s mixing guide covers what order to mix in, checking salinity and preparing its own salts. Those instructions are for Red Sea salts; they don’t back up the made-up ratio above or apply to every brand. Follow the current instructions for your own salt, and check every batch.
Save your plan and track the spending
Save your mixing plan on this device, then import its yearly salt and water costs into the running cost calculator. Importing again replaces the linked lines rather than adding more. Only add a separate line for mixing equipment or power if it isn’t already counted.
Use the Companion to record the packs and water you actually buy. Note the exact salt and where your ratio came from in your plan, and keep a downloaded backup. Update the price when you buy again; an old offer isn’t a lasting running cost.
Common questions
Is ppt the same as grams of salt per litre?
No. The planner records your target salinity separately from your salt’s grams-per-litre ratio. Enter a ratio from the maker or one you’ve checked yourself.
Are water and electricity included?
The planner includes the water price per litre you enter. It leaves out power and equipment for mixing; add those to the running cost calculator without counting them twice.
Why is the number of batches a year not a whole number?
The yearly cost uses 365 divided by your interval. How many changes fall in a particular month is a separate calendar question.
Sources and limits
Last checked 12 October 2026. Our worked examples use made-up inputs to show the maths. They aren’t measured costs or care targets for your tank.
Keep going
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