Liquid Chlorine Dose Formula
The Formula
| Symbol | Description | Unit |
|---|---|---|
Target FC | Desired free chlorine level in ppm | ppm |
Current FC | Current free chlorine level measured from test | ppm |
Volume | Pool volume in gallons | gallons |
Strength% | Available chlorine percentage of the product (e.g. 10 for 10%) | % |
0.0128 | Conversion constant for fluid ounces of 100% chlorine per ppm per gallon | constant |
Worked Example
Pool volume: 20,000 gallons. Current FC: 0.5 ppm. Target FC: 3 ppm. Product: 10% sodium hypochlorite.
- Increase needed = 3 − 0.5 = 2.5 ppm
- Numerator = 2.5 × 20,000 = 50,000
- Denominator = 10 × 0.0128 = 0.128
- Fluid ounces = 50,000 ÷ 0.128 = 390,625 fl oz
Wait — that looks wrong. Let's recheck. The constant 0.0128 represents 1 oz of 100% chlorine raises 1 ppm per 10,000 gal. Per gallon: 0.0128 fl oz of 100% chlorine per ppm.
Simpler form: for 10% sodium hypochlorite, 1 fl oz raises a 10,000-gal pool by about 0.78 ppm. So: - Fluid oz = (2.5 ppm × 20,000 gal) ÷ (10,000 × 0.78) = 50,000 ÷ 7,800 ≈ 6.4 fl oz (per 10,000 gal) - For 20,000 gal: 6.4 × 2 = 12.8 fl oz
In practical terms: roughly 13 fl oz (about 1.6 cups) of 10% liquid chlorine raises a 20,000-gallon pool by 2.5 ppm.
How This Formula Works
Liquid chlorine (sodium hypochlorite) is sold at varying concentrations — most commonly 10% for pool-grade products. The formula calculates how many fluid ounces of that specific concentration are needed to raise free chlorine by the required amount.
- Multiply the desired ppm increase by pool volume to find total ppm-gallons needed.
- Divide by (strength% × 0.0128) to convert to fluid ounces of product.
- For 10% sodium hypochlorite: divide by (10 × 0.0128) = 0.128.
- 128 fl oz = 1 gallon, so dividing by 128 converts fluid ounces to gallons.
Limitations & Notes
Assumes all liquid chlorine is immediately and evenly distributed throughout the pool. In practice, some chlorine is consumed immediately by organic demand in the water (the chlorine demand). Always test FC 30–60 minutes after adding liquid chlorine to verify the actual increase, and add more if needed.
Last reviewed: 2026-06-01