Quick calculators for pipe sizing and water balance.

Pipe sizing on a pool comes down to water velocity — how fast water moves through the pipe, in feet per second. Push water too fast and you get noise, higher head loss (so the pump works harder and uses more energy), and, over years, erosion at fittings and elbows. Size the pipe too generously and you have spent money on material and trenching you did not need.

The common working limits for rigid PVC are roughly 6–8 ft/s on pressure (return) lines and 6 ft/s or less on suction lines, where cavitation and pump strain are the concern. The three calculators here let you solve for whichever value you are missing:

• Find Flow — you know the pipe size and a target velocity, and want the resulting GPM.
• Find Velocity — you know the pipe size and the actual flow, and want to check whether the water is moving too fast.
• Find Pipe Size — you know the design flow and a velocity ceiling, and want the smallest pipe that stays under it.

SCH40 vs SCH80: both share the same outside diameter for a given nominal size, but SCH80 has thicker walls and therefore a smaller inside diameter, so the same flow runs faster through SCH80. SCH80 is used where pressure ratings or mechanical strength require it, most often on the equipment pad.

The Langelier Saturation Index (LSI, sometimes just “SI”) estimates whether pool water is chemically balanced with respect to calcium carbonate. It combines five inputs — pH, water temperature, calcium hardness, total alkalinity, and (optionally) cyanuric acid — into a single number.

• Negative (below −0.3): the water is corrosive. It will pull calcium out of plaster, grout, and concrete, etch surfaces, and attack metal at heaters and fittings.
• Positive (above +0.3): the water is scale-forming. Calcium carbonate drops out of solution as cloudy water, rough deposits on surfaces, and scale inside the heater and plumbing.
• Between −0.3 and +0.3: balanced. Zero is ideal.

Because the index is additive, there is more than one way to fix an out-of-range result: a pool running hot in summer can offset that temperature factor by holding pH and alkalinity slightly lower. Adjust one factor at a time, re-test, and recalculate. The cyanuric acid field matters because stabilizer contributes to the total alkalinity reading; this tool subtracts an approximate CYA share so the alkalinity factor reflects carbonate alkalinity, which is what actually drives the index.

Enter your most recent test-kit numbers to start, then use the calculator to see which single change moves you closest to zero.

Gas pool heaters are rated two ways, and the difference matters when you read these results. Input (KBtu/hr) is the fuel the heater consumes and is the number printed on the rating plate and used to size the gas line. Output is the heat actually delivered to the water — input multiplied by combustion efficiency, typically 82–84% for standard units and higher for low-NOx and condensing models.

Which method to use

By surface area answers “what continuous output do I need to hold a temperature?” Most heat leaves a pool through its surface — evaporation first, then radiation and convection — so surface area and exposure drive the steady-state load. Use this for a pool kept warm through the season.

By volume and heat-up time answers “how big a heater do I need to warm the pool from cold within X hours?” It is governed by the mass of water: about 8.34 Btu raises one gallon one degree Fahrenheit. Use this for a spa or a pool that gets heated on demand rather than held warm.

Real-world requirements climb with wind exposure, low overnight temperatures, humidity, and an uncovered surface. A pool cover is the single biggest lever — it can cut heat loss by roughly half. When two methods disagree, size to the larger result and round up to the next standard heater.