At Grade Pool — T40F Sizing & Component Calculator
Size your T40F filter tank and skimmer components in a few quick steps.
Follow Steps A → FRectangular Contracted Weir Calculator
Enter the waterfall width (L) and the water depth flowing over the falls (H) to calculate
the waterfall GPM. Formula: Q = 3.33 × (L − 0.2H) × H3/2
🛠️Build Your System
Your sizing results are pre-loaded below. Choose a skimmer series and model, adjust quantities, then add the whole system to your cart.
Adds all selected items to your Filtrific Store cart. You can adjust quantities or remove items at checkout. Free shipping on all orders.
Size your whole system in a few friendly steps
Tell us about your waterfall and pool, and this water feature calculator does the math — total flow in GPM, the right number of skimmers, and a matching Filtrific® filter tank you can add straight to your cart. No guesswork, no spreadsheets.
How this calculator works
Everything runs off two things you can measure on site. Here's the flow:
Measure the falls
Enter your waterfall width and how deep the water runs over the edge.
Add the pool
Pop in the surface area and we work out the circulation your water needs.
Get your build
See total GPM, skimmer count, filter tank — ready to add to cart.
Why flow rate comes first
A water feature only performs as well as the water moving through it.
Two flows matter, and this calculator adds them together to find the total your system must handle. The first is your waterfall flow — the water spilling over your falls or weir. The calculator estimates it from the width of the waterfall and the depth of water flowing over it, using a standard rectangular contracted weir equation, the same physics that governs how water moves over any horizontal edge. The second is circulation flow — the turnover that keeps a pool or basin clean and moving. The tool figures this at roughly 0.15 GPM for every square foot of water surface, so a larger pool draws more circulation.
Add the two and you have your total waterflow in GPM — the single most important number for choosing a pump, a filter tank, and the skimmers that feed them. Undersize it and the falls look thin while debris lingers on the surface; oversize it and you pay for pump energy you don't need. Getting this number right first is what separates a feature that runs trouble-free for years from one that fights its owner.
Sizing skimmers for a clean surface
Skimmers pull water — and floating leaves, pollen, and debris — off the surface before it can sink and decay.
Each skimmer can only move so much water, so the number you need depends on your total flow. Filtrific's low-profile, stainless-steel skimmers are built to disguise the hardware and keep the focus on the design rather than a bulky weir or skimmer box. They come in two flow classes this calculator sizes for:
Ideal for smaller streams, spillways, and fountains where a discreet weir blends into the wall.
Built for higher-flow waterfalls, with a wider weir that moves more water per unit.
The calculator divides your total flow by each series' capacity and rounds up, so you always land on a whole number of skimmers that comfortably carries the load. It also reports the depth of water that will flow over each skimmer's weir, which helps you confirm the finished look before anything is installed.
Matching the right filter tank
At the heart of a Filtrific system is the filter tank — a heavy-duty HDPE vault that does several jobs in one unit.
It combines filtration, water-in-transit storage, and a housing for the pump and pumping hardware. Because it consolidates those jobs, it reduces both system complexity and the mechanical footprint in the landscape or utility room, and it routinely costs less than a custom concrete or fiberglass pump vault. Here's what makes the tank the anchor of your build:
Built-in ports
Pre-built inlet and outlet ports adapt to complex piping — no custom fabrication.
Built-in pre-filtration
An in-tank basket with multiple mesh options, cleaned from the top through a removable cap.
Large pump chambers
Room for big or multiple pumps, plus auto-fill and pump shut-off add-ons.
Expandable reservoir
40, 75, or 390 gallons built in, with expansion tanks when a design needs more.
This particular calculator sizes an at-grade pool built around the T40F filter tank, which handles systems up to 60 GPM. If your total flow comes in higher than that, the tool tells you so and points you toward a larger tank — the same conservative sizing a Filtrific specialist would recommend rather than pushing a tank past its rating.
Quick heads-up on the math: every result here is an estimate to guide your design. Site conditions — head height, pipe runs and fittings, elevation changes, and the specific pump curve — all affect real-world performance. Use these figures to get into the right range, then confirm the final build with a professional.
Common questions
How many gallons per minute does my waterfall need?
It depends on how wide your falls are and how full a sheet of water you want. Enter the waterfall width and the depth of water flowing over the edge, and the calculator returns the waterfall GPM for you. A wider or deeper flow moves more water and calls for a larger pump and filter tank.
How many skimmers do I need for my water feature?
Divide your total system flow by the capacity of the skimmer you're using — 25 GPM for the S2 Series or 60 GPM for the S3 Series — and round up. The calculator does this automatically and shows the count for both series so you can compare.
Can I hide the filter tank and hardware?
Yes. Filtrific filter tanks are designed to be buried or placed remotely — even in a utility room or parking garage away from the feature — and the low-profile, stainless-steel skimmers and returns are made to disguise the hardware so the water, not the equipment, stays the focal point.
What if my flow exceeds the T40F's limit?
The T40F is rated to 60 GPM. If your total waterflow lands above that, this calculator flags it and directs you to a larger tank so the system isn't run past its rating.
Still weighing your options?
Filtrific's sizing specialists will help you dial in the right tank, skimmers, and components.