Maintaining the right chlorine level in your swimming pool is crucial for sanitation and swimmer comfort. However, sometimes chlorine levels can become excessively high due to over-dosing, shock treatments, or other factors. High chlorine can be irritating to skin and eyes, damage pool equipment, and even interfere with other chemical readings.
Sodium thiosulfate is a common and effective chemical used to quickly lower chlorine levels in pool water. It acts as a dechlorinator, neutralizing free chlorine through a chemical reaction. This calculator helps you determine the precise amount of sodium thiosulfate needed to bring your pool's chlorine back to a safe and desired range.
How to Use the Sodium Thiosulfate Calculator
To get an accurate calculation, you'll need three key pieces of information:
Pool Volume (Gallons): This is the total amount of water in your pool. You can often find this in your pool's documentation, or you can calculate it based on your pool's dimensions (e.g., Length x Width x Average Depth x 7.5 for rectangular pools).
Current Chlorine Level (ppm): Test your pool water using a reliable test kit or strips to determine the current free chlorine concentration in parts per million (ppm).
Desired Chlorine Level (ppm): This is the target chlorine level you want to achieve. For most residential pools, a free chlorine level of 1-3 ppm is considered ideal.
Sodium Thiosulfate Purity (%): Sodium thiosulfate is often sold as a granular powder (typically 100% pure) or as a liquid solution (which will have a lower percentage purity). Check the product label for this information. If you're using a granular product and the label doesn't specify, assume 100%.
Enter these values into the calculator below, and it will tell you how much sodium thiosulfate (in both ounces and grams) you need to add.
Important Considerations When Using Sodium Thiosulfate
Add Slowly and Retest: It's always best to add a portion of the calculated amount, allow the pool water to circulate for a few hours, and then retest the chlorine level. You can always add more if needed.
Don't Overdo It: Adding too much sodium thiosulfate can completely deplete your chlorine, leaving your pool vulnerable to algae and bacteria. If you accidentally add too much, you'll need to re-chlorinate your pool.
Temporary Fix: While effective, sodium thiosulfate is a temporary solution. If your chlorine levels are consistently too high, investigate the root cause (e.g., faulty feeder, improper dosing, high CYA levels).
Safety First: Always handle pool chemicals according to manufacturer instructions. Wear appropriate protective gear and store chemicals safely.
Water Chemistry Balance: Sodium thiosulfate can slightly lower pH, but typically not significantly enough to cause major issues unless very large amounts are used. Always monitor your overall water chemistry.
.calculator-container {
background-color: #f9f9f9;
border: 1px solid #ddd;
padding: 20px;
border-radius: 8px;
max-width: 500px;
margin: 20px auto;
font-family: Arial, sans-serif;
}
.form-group {
margin-bottom: 15px;
}
.form-group label {
display: block;
margin-bottom: 5px;
font-weight: bold;
}
.form-group input[type="number"] {
width: calc(100% – 22px);
padding: 10px;
border: 1px solid #ccc;
border-radius: 4px;
box-sizing: border-box;
}
button {
background-color: #007bff;
color: white;
padding: 12px 20px;
border: none;
border-radius: 4px;
cursor: pointer;
font-size: 16px;
width: 100%;
box-sizing: border-box;
}
button:hover {
background-color: #0056b3;
}
.result-container {
margin-top: 20px;
padding: 15px;
border: 1px solid #e0e0e0;
border-radius: 4px;
background-color: #e9f7ef;
font-size: 1.1em;
font-weight: bold;
color: #333;
}
.error-message {
color: #d9534f;
font-weight: bold;
}
.small-text {
font-size: 0.8em;
font-weight: normal;
color: #555;
}
function calculateST() {
var poolVolume = parseFloat(document.getElementById('poolVolume').value);
var currentChlorine = parseFloat(document.getElementById('currentChlorine').value);
var desiredChlorine = parseFloat(document.getElementById('desiredChlorine').value);
var stPurity = parseFloat(document.getElementById('stPurity').value);
var resultDiv = document.getElementById('result');
// Clear previous results
resultDiv.innerHTML = ";
// Input validation
if (isNaN(poolVolume) || poolVolume <= 0) {
resultDiv.innerHTML = 'Please enter a valid Pool Volume (must be a positive number).';
return;
}
if (isNaN(currentChlorine) || currentChlorine < 0) {
resultDiv.innerHTML = 'Please enter a valid Current Chlorine Level (must be 0 or greater).';
return;
}
if (isNaN(desiredChlorine) || desiredChlorine < 0) {
resultDiv.innerHTML = 'Please enter a valid Desired Chlorine Level (must be 0 or greater).';
return;
}
if (isNaN(stPurity) || stPurity 100) {
resultDiv.innerHTML = 'Please enter a valid Sodium Thiosulfate Purity (between 1 and 100%).';
return;
}
if (currentChlorine <= desiredChlorine) {
resultDiv.innerHTML = 'Your current chlorine level is already at or below your desired level. No sodium thiosulfate is needed.';
return;
}
var chlorineReduction = currentChlorine – desiredChlorine;
// Calculation based on the common rule of thumb: 1 ounce of pure sodium thiosulfate reduces 1 ppm in 10,000 gallons.
var pureSTNeededOz = (chlorineReduction * poolVolume) / 10000;
// Adjust for purity
var actualSTNeededOz = pureSTNeededOz / (stPurity / 100);
// Convert to grams (1 oz = 28.35 grams)
var actualSTNeededGrams = actualSTNeededOz * 28.35;
resultDiv.innerHTML =
'To lower your chlorine from ' + currentChlorine.toFixed(1) + ' ppm to ' + desiredChlorine.toFixed(1) + ' ppm, you will need:' +
'' + actualSTNeededOz.toFixed(2) + ' ounces of ' + stPurity.toFixed(0) + '% Sodium Thiosulfate' +
'or approximately ' + actualSTNeededGrams.toFixed(2) + ' grams of ' + stPurity.toFixed(0) + '% Sodium Thiosulfate' +
'(Always add slowly, retest, and adjust as needed.)';
}