poolchemistrycalc

Hot tub chemical calculator

A 6 seat tub holds

330gal 1,250L

First thing out of band: Free chlorine at 0.5 ppm, which takes 1.0 fl oz of HASA CHLOR. Seats and steps take the rest: 32% of the shell is water.

Section through a rectangle tub 84 in across and 34 in deep, with the 330 gal it holds drawn as the 10.9 in a bare box of the same footprint would fill to. 84 in × 84 in 34 in shell, rim to floor not the depth used 10.9 in of water, spread flat gallons use this 330 gal 32% water

Your tub and your readings

how many people it seats
inches, outside edge to outside edge
inches
inches, rim to floor
in ppm
in pH
in ppm
in ppm
in ppm

The answer, the drawings, every reading and every table on this page are worked for the values in these fields, and worked again as you type. The questions further down are answered for the example this page was published with.

At 330 gal

What to add

Every product that moves a reading outside its band, worked at the tub's gallons. A pool dose in a tub is roughly fifty times too much.
Fixes Product Add
Free chlorine
+3.0 ppm 		HASA CHLOR
sodium hypochlorite		 1.0 fl oz
30 mL
Free chlorine
+3.0 ppm 		Multi-Chlor
sodium hypochlorite		 1.0 fl oz
30 mL
Free chlorine
+3.0 ppm 		Pool Shock Calcium Hypochlorite 65%
calcium hypochlorite		 0.2 oz
6 g
pH
−0.5 pH 		HASA Dry Acid
sodium bisulfate		 0.7 oz
19 g
pH
−0.5 pH 		HASA Muriatic Acid
hydrochloric acid		 no quantity — see the label

Alkalinity buffers pH, so set the alkalinity first — the pH you correct before it will move again when the alkalinity does. Add one thing at a time, let the pump turn the water over, and test again before you add the next. No product page we have read states how long to wait, so we do not put a number on it.

The bands for a tub

Reading Range
pH
COMAR 10.17.01.45 A(1) 		7.2–7.8 pH
Free chlorine
Florida 64E-9.004 (1)(d)2 		2–5 ppm
Total alkalinity
COMAR 10.17.01.45 A(2) 		60–180 ppm
Calcium hardness
COMAR 10.17.01.45 A(3) 		150–400 ppm
Stabiliser
Florida 64E-9.004 (1)(d)4 		up to 40 ppm
Water temperature
COMAR 10.17.01.45 C(2) 		up to 104 °F

A hot tub holds a fraction of a pool and heats the water, so pool quantities are wrong here. The volume here is worked out from five manufacturers' published shell sizes and water capacities and fits a moulded acrylic tub; if yours is inflatable, use the capacity printed on its box. Never mix chemicals, and never add anything while anyone is in the water.

What people ask

What are the normal chemical readings for a hot tub?
ph 7.2–7.8 pH; free chlorine 2–5 ppm; total alkalinity 60–180 ppm; calcium hardness 150–400 ppm; stabiliser up to 40 ppm, each quoted from the code named beside it on this page. A tub is not a small pool: the same reading takes a fraction of the quantity, because 330 gal is what it holds.
What is the order of adding chemicals to a hot tub?
Alkalinity, then pH, then sanitiser, one at a time with the pump running. Test again before the next. No product page we have read states how long to wait, so we do not put a number on it. Never add anything while anyone is in the water.

Fix a reading

Updated September 2026

Pool and spa rules are adopted and amended state by state, and most of the ranges here are written for public pools. Confirm every figure against the code your jurisdiction has adopted and against your own product label.

Sources and dates

Ranges are quoted from the code named beside them, product strengths from the manufacturer's own product page, each retrieved on the date shown.