Pool Calculator uses Volume = length × width × average depth, or π × radius² × depth, to estimate pool gallons or liters, fill time, salt, chlorine dose, and heating energy from your pool size inputs.
Four Numbers, One Volume
Every preparation task for a swimming pool — filling it, salting it, shocking it, heating it — traces back to the same single figure: how many gallons the pool actually holds. People typically look these up separately and end up with scattered estimates from different sources. This calculator derives all four outputs from the same volume calculation so they’re consistent with each other and with your actual pool dimensions.
Enter your pool’s length, width, and average depth, and it returns fill time, the salt quantity needed to reach generator-ready concentration, a liquid chlorine shock dose, and the heating energy required per degree of temperature rise. All in one pass.
How Each Output Is Derived
Volume is the foundation. For rectangular pools, the formula is length × width × depth in feet, converted to gallons using the standard factor of 7.480519 gallons per cubic foot. Circular pools use π × radius² × depth instead — the width field disappears automatically in that mode because a circle’s area is fully defined by its diameter alone. In Metric mode, the cubic meter result is multiplied by 1,000 to produce liters.
Fill time assumes a standard garden hose flowing at 9 gallons per minute (34 liters per minute in metric). The calculation is straightforward: total gallons divided by that flow rate gives total minutes, divided by 60 to get hours. Municipal water pressure varies, hose diameter varies, and running other fixtures at the same time drops the flow — so treat this as a planning estimate, not a countdown timer. Most homeowners find the actual fill runs 10–20% longer than the figure shown.
Initial salt is calculated to reach a target concentration of 3,200 ppm — the middle of the operating range for most residential salt chlorine generators. The formula works out to approximately 0.0267 lbs of salt per gallon of pool water (or 0.0032 kg per liter), which is the equivalent of 3,200 milligrams of dissolved salt per liter. The bag count assumes standard 40-lb bags in US mode and 25 kg bags in metric. This output is specific to salt pools — it’s not a general pool salt treatment amount.
Chlorine shock is sized for 12.5% sodium hypochlorite, which is the concentration sold at pool supply stores as “liquid chlorine” or pool-grade bleach (not the 3–6% found in household bleach). The dose targets a +10 ppm increase in free chlorine — a standard opening shock level. The US result shows gallons of 12.5% product alongside the fluid-ounce figure; metric shows liters and milliliters.
Heating energy uses the specific heat of water: raising one pound of water by 1°F requires exactly 1 BTU. Since a US gallon of water weighs 8.34 lbs, each gallon needs 8.34 BTU per degree of temperature rise. The calculator applies this per 1°F (or 1°C in metric) and then shows how long a 100,000 BTU/h gas heater (or 30 kW electric heater in metric) would theoretically take to deliver that energy. Read the next section before relying on the heating figure.
The Heating Output Only Covers One Degree — Here’s Why That Matters
Most pool owners expect a heating calculator to ask “what’s your current water temperature?” and “what do you want it to reach?” This one doesn’t. The calculator outputs BTUs required for a single degree of rise because starting temperature, desired temperature, overnight heat loss, and ambient conditions are all variables it doesn’t have access to.
To use the heating result for your actual situation: note the per-degree BTU figure, then multiply by the temperature difference you need. If your pool is sitting at 62°F and you want it at 82°F, multiply the BTU result by 20. Divide the total by your heater’s output (in BTU/h) to get an ideal no-loss run time. Real run time will be longer because heaters cycle, pools lose heat to the air overnight, and gas pressure affects burner output. The figure shown is theoretical, not a schedule.
In metric mode, the same logic applies but the output is in kilowatt-hours per 1°C of rise, with a 30 kW reference heater shown for scale.
Worked Example: Prepping a New In-Ground Pool for First Fill
A pool service company was handed a new 24 × 12 ft rectangular gunite pool with an average depth of 4.5 feet (shallower shelf entry at 3 ft, deep end at 6 ft, averaged). The owner was adding a salt chlorine generator and wanted to know exactly what to order before the fill date. Running those dimensions in US Customary mode:
- Pool volume: 1,296 cubic feet → 9,694 US gallons
- Fill time at 9 GPM: 1,077 minutes → just under 18 hours (they started the hose before lunch and checked it the next morning)
- Salt to reach 3,200 ppm: 258.8 lbs → 6.5 bags of 40 lbs each (they ordered 7 bags)
- Opening shock at 12.5% liquid chlorine: 103.7 fl oz → about 0.81 gallons of product
- Heating per 1°F: 80,854 BTU — the pool filled in early May at approximately 58°F, target was 80°F, so they multiplied by 22°F = 1,778,788 BTU total, roughly 17.8 hours on their 100k BTU/h propane heater under ideal conditions
Having all four numbers before the fill meant they showed up with the right supplies. The salt figure in particular saved a second trip — most people guess low on salt for a first fill and end up short of the generator’s minimum operating range.
Frequently Asked Questions
My salt chlorine generator targets 2,700 ppm, not 3,200. How do I adjust the salt figure?
The 3,200 ppm target is fixed in the calculator’s formula. To get the right amount for a different target, scale the result proportionally. For 2,700 ppm, multiply the displayed salt quantity by 2700 ÷ 3200 = 0.844. For a target of 3,500 ppm, multiply by 3500 ÷ 3200 = 1.094. Most generator manufacturers specify a range of 2,700–3,400 ppm; 3,200 ppm is a common midpoint default but check your unit’s manual before dosing.
My liquid chlorine is 10% strength, not 12.5%. Is the shock figure still usable?
Not directly — the shock formula is calibrated for 12.5% sodium hypochlorite specifically. If your product is 10%, multiply the calculated volume by 1.25 to get the equivalent dose. For household bleach at 6%, multiply by approximately 2.08. Never use the figure as-is with a different concentration; chlorine dose errors affect both safety and water quality.
The width field disappears when I switch to Circular — is the calculator ignoring my input?
That’s intentional behavior, not a missing field. A circle’s area is fully determined by diameter alone, so width has no role in the calculation. Enter your pool’s full diameter (rim to rim at the widest point) in the length/diameter field and leave it there. If your round pool is actually oval, use the rectangular mode with length and width as your two longest measurements and accept that the result will be a ceiling estimate.
What assumptions does the fill-time estimate make that might throw it off in practice?
Three main ones: the calculator assumes a constant 9 GPM hose flow rate, a completely empty pool, and no interruptions. In practice, municipal supply pressure fluctuates throughout the day, longer hose runs reduce flow, and leaving a hose running overnight in cold weather can slow things further. If you’re filling a pool that held water and just needs topping off, the total gallon count will obviously be less than the full-pool figure — there’s no partial-volume input. Use the displayed total to calculate your own fill time by proportion.