Reef Return Pump Calculator

Why the box GPH is a lie

Every return pump is rated at its maximum flow with zero head — pump sitting next to a bucket, no plumbing. The moment you lift water up to your display and push it through pipe, elbows, and a return nozzle, flow drops fast. A pump rated "1000 GPH" can easily deliver 500–600 at a typical 4-foot rise. Sizing off the box number is the single most common return-pump mistake.

The three numbers that actually size a pump

1. Target flow (turnover). Most reef tanks run their return at 5–10× the display volume per hour. Mixed reefs are happy around 7×; SPS-heavy tanks push higher; fish-only can go lower. This is flow at the tank, after losses.

2. Static head. The vertical distance from your sump's water surface up to the return outlet. Every foot of rise costs flow.

3. Friction (dynamic) head. Losses from pushing water through pipe and fittings. Narrower pipe and more elbows = dramatically more loss. We compute this with the Hazen-Williams equation (C=150 for PVC) plus an equivalent-length allowance for each 90° elbow.

Total dynamic head, explained

Add static head + friction head and you get total dynamic head (TDH) — the real resistance your pump fights. We find each pump's operating point (where its flow-vs-head curve crosses your system's curve) and report the flow it truly delivers there. A pump "makes your target" only if that delivered flow meets your turnover goal.

Should you oversize?

A little, yes — pick the smallest pump that clears your target, then confirm it has a bit of headroom (and a gate valve to dial it back). Massively oversizing wastes energy, adds heat and noise, and often gets throttled down anyway. DC pumps make this easy: they're adjustable and quieter, usually worth the premium over AC for a return.