Garden & outdoor
How much chlorine does your pool need?
Free pool chlorine calculator: grams and tablets for maintenance, shock or season opening from pool volume in m³.
Quick answer
Pool chlorine is dosed per cubic metre of water, not per bag on instinct: chlorine (g) = volume (m³) × dose (g/m³). Maintenance and shock are two different jobs, with different products and different days.
Advanced settings
Active chlorine needed
80 g
- Product to add
- 89 g
- Tablets (rounded up)
- 1
- Water volume used
- 40,000 L (40 m³)
⚠️ Indicative doses only — always follow the product label and never mix pool chemicals together.
Formula: chlorine (g) = volume (m³) × dose (g/m³). Tablets = round up to active chlorine ÷ (tablet weight × chlorine %). Target free chlorine 1–3 ppm for maintenance; shock after rain, algae or heavy use. Test pH (7.2–7.6) before dosing.
How it works
| Job | Dose | On a 40 m³ family pool |
|---|---|---|
| Weekly maintenance | 2 g/m³ | 80 g active → about one 200 g tablet at 90% |
| Shock, after green water or heavy rain | 10–15 g/m³ | up to 480 g active chlorine |
Shock vs granular vs liquid — they are not interchangeable: Shock (often calcium hypochlorite or dichlor granules labelled “shock”) is a high one-off dose (10–15 g/m³) to clear algae or after a storm — wait until free chlorine falls back to 1–3 ppm before swimming. Granular chlorine (fast-dissolve) is for measured top-ups and shocks; pre-dissolve in a bucket of pool water, never dump dry on the liner. Liquid chlorine (sodium hypochlorite, bleach-strength pool grade) acts immediately and adds no cyanuric acid — good for frequent small doses, but it loses strength in storage and can raise pH. Slow trichlor tablets are for weekly maintenance in a floater/skimmer, not for shock.
Start from your volume: if you only know length and width, use our pool volume calculator first. Then pick maintenance, shock or season opening, adjust product % and tablet size in advanced settings, and verify with pool test strips — free chlorine 1–3 ppm, pH 7.2–7.6.
Check the pH before the dose, not after: above 7.6 chlorine works at roughly half strength, so you keep adding product to water that cannot use it. Add after sunset with the filter running — ultraviolet light destroys free chlorine during the day, and a midday dose is largely wasted. If the water has already turned green you are past prevention: a shock dose and patience, and the calculator above is for maintenance again from the next week. All of it starts from the volume — if you are not sure of yours, the pool volume calculator has a way to measure it from the water meter that works even on an irregular shape.
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Frequently asked questions
How much chlorine for green pool water?+
Green water needs shock chlorination, not maintenance tablets: typically 10–15 g/m³ of active chlorine (select Shock in the calculator), brush walls, run the pump 24–48 h, then backwash the filter. Correct pH to 7.2 first — high pH makes chlorine ineffective. If it greens again within days, test cyanuric acid stabilizer.
Shock vs granular vs liquid chlorine?+
Shock = high one-time dose to clear problems. Granular = measured powder for top-ups or shock (pre-dissolve). Liquid = fast, no stabilizer, short shelf life. Tablets = slow weekly maintenance only. Enter your product % in advanced so grams of product match active chlorine.
Tablets or granular chlorine?+
Slow trichlor tablets (200 g, ~90% active) suit steady maintenance in a dispenser. Fast-dissolving granules or shock powder are for superchlorination — never crush tablets into the skimmer for a shock dose. Enter your product's chlorine % in advanced settings; multi-action tablets often show lower active % than pure trichlor.
How do I know my pool volume in m³?+
Rectangular: length × width × average depth. Round: π × (diameter ÷ 2)² × depth. Average depth is halfway between shallow and deep end. A 7 × 3.5 m pool at 1.4 m average depth holds about 34 m³ (34,000 L). Use the dimensions mode here or the dedicated pool volume calculator for pump sizing too.
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