Measuring for a 20×40 pool cover requires the A-B triangulation method, which uses two fixed stakes and a series of perimeter measurements to map the exact shape of your pool.
Ordering a custom safety cover for your 20×40 inground pool isn’t as simple as noting its length and width. Pools settle, corners shift, and obstructions like handrails or rock features throw off a basic rectangle. If you skip the proper measurement process, you’ll end up with a cover that sags, gaps, or fails to secure. The industry-standard method—A-B triangulation—maps every contour precisely so the cover arrives tight and safe. Here is exactly how to do it, step by step, with the real measurements manufacturers need.
Why The A-B Triangulation Method Is Mandatory
A straight 20×40 measurement assumes a perfect rectangle, but no pool stays perfectly square over years of ground movement. The A-B method uses two fixed reference stakes to record coordinates for every point around the pool, letting the manufacturer build a cover that matches the actual shape—curves, offsets, and all. Manufacturers like Pool Guard and In The Swim require these triangulated measurements, not simple dimensions.
Without this method, the most common failure is a cover that’s twisted or too small across one corner. The two stakes create a fixed baseline that every measurement traces back to, eliminating guesswork.
Step 1: Set Up Your A-B Reference Line
Place two thin stakes—or long screwdrivers—parallel to the pool’s longest side, at least 4 feet from the water’s edge and roughly 20 feet apart (about two-thirds of the pool’s length). Label the left stake “A” and the right stake “B.”
- Ensure the imaginary line between A and B does not cross the pool at any point.
- Position the line at least 3–4 feet behind any curved section of the pool.
- Drive stakes firmly into the ground so they don’t shift during measurement.
- Measure the exact A-B distance with a reel-type tape and record it—you’ll need this for the scaled sketch.
Step 2: Mark Your Perimeter Points Clockwise
Walk the inside edge of the pool coping and place a mark every 24–36 inches on straight sections. On curves and tight radiuses, place marks every 6–12 inches. Number each point clockwiSE starting at 1. Mark every fifth point (1, 5, 10, 15) with a bolder mark so you don’t lose your place.
Also mark every change in direction—corners, steps, and any spot where the coping angle shifts. If you have handrails, ladders, or rockwork within 2–3 feet of the waterline, mark those locations as separate points too. Measuring counterclockwise will produce a mirror-image cover that won’t fit, so verify your direction twice before proceeding.
Step 3: Take The A-B Triangulation Measurements
Using a 100-foot reel tape, attach the end to stake A. Walk to Point 1, pull the tape taut, and record the distance from A to Point 1 to the nearest inch. Move to Point 2, Point 3, and so on, recording each A-to-point distance.
Once all A measurements are done, move the tape to stake B and repeat the process: B to Point 1, B to Point 2, and so on around the entire perimeter. Each point now has a coordinate pair (A1, B1; A2, B2; A3, B3).
| Measurement Round | Stake Used | Points Measured |
|---|---|---|
| First round | Stake A | 1, 2, 3, 4, 5, 6… |
| Second round | Stake B | 1, 2, 3, 4, 5, 6… |
| Verification | Cross-pool diagonals | Point A to opposite corner, B to opposite corner |
| Check lines | Opposite perimeter points | 3–4 cross-pool distances between non-adjacent points |
Step 4: Verify With Cross Dimensions
Accuracy checks prevent costly mistakes. Measure 1–2 long diagonals across the pool (e.g., from the deepest corner to the farthest opposite corner) and 1–2 wide diagonals. Take 3–4 cross-pool measurements between opposite perimeter points. If any cross-dimension conflicts with the triangulation data, re-measure those points before submitting.
A loose tape is the most common source of error here. Pull every measurement taut and have a helper hold the stake end steady.
Step 5: Account For Obstructions And Water Drop
Non-removable features within 2–3 feet of the waterline change the cover dimensions.
These additions ensure the cover reaches past the obstruction and sits flat on the water or ground beneath it.
Step 6: Create Your Scaled Sketch
Draw the A-B line to scale on graph paper. Plot each numbered point using its A and B distances. Connect the points to form the pool perimeter. Add the A-B distance, the cross-dimension lines, and the location of all obstructions. Include your name, pool dimensions, and any notes about steps or special features. A clean sketch cuts down on back-and-forth with the manufacturer and gets your cover delivered faster.
If you’re ready to compare cover options and pricing, our top 20×40 pool cover recommendations break down the best models for different budgets and priorities.
| Cover Type | Typical Price Range (2024–2026) | Best For |
|---|---|---|
| Custom mesh safety cover (ASTM certified) | $1,200–$2,500 | Child/pet safety, debris drainage |
| Standard winter cover (non-safety) | $150–$400 | Light seasonal debris, budget protection |
| Heavy-duty solid vinyl safety cover | $1,800–$3,200 | Leaf-heavy areas, total sunlight block |
| Economy mesh safety cover | $800–$1,400 | Budget safety with basic drainage |
Finish With Your Final Measurements
Once you’ve measured all points, verified accuracy with cross dimensions, and added the obstruction or water-drop allowances, round up to the next standard cover size if your numbers don’t match a stock option exactly. Submit your A-B diagram and measurement form to the manufacturer—either as a print form or a digital PDF. The final dimensions they produce will include a built-in 1-foot overlap per side, so you do not need to add extra material for overlap yourself.
Owner-installed covers benefit from adding landscape rocks or weight bags across the cover surface during the first setup, especially on windy days.
FAQs
What happens if I measure my pool counterclockwise?
Measuring counterclockwise produces a mirror-image cover that will be reversed when installed. The points and coordinates will map a flipped version of your pool, causing corners and cutouts to sit opposite where they should. Always confirm clockwise direction before recording your first point.
Do I need to measure obstructions like handrails separately?
Yes. Any feature within 2–3 feet of the waterline—handrails, ladders, rocks, fixed slides—must be marked as separate points and reported to the manufacturer. The cover must extend past these obstructions, so you add twice the height of each obstacle to both the length and width dimensions.
Can I use a laser distance measurer instead of a tape?
Most manufacturers still recommend a 100-foot reel-type measuring tape because it maintains constant tension and doesn’t drop signal near water or reflective pool finishes. Laser measures can drift over long distances outdoors in sunlight. A tape is more reliable for this method.
How do I know if my measurement is accurate enough?
After taking all A-B pairs, measure 3–4 cross-pool diagonals between opposite perimeter points. If those diagonal lengths conflict with what your triangulation coordinates predict, re-measure the mismatched points. One incorrect reading can throw off the entire cover shape.
Will a 20×40 cover fit if my pool has slight curves?
Yes, if you use the A-B method to map every curve point at 6–12 inch intervals. A cover built from coordinate data will follow the real shape. A simple rectangular 20×40 cover ordered without coordinate mapping will not conform to the curves and will leave gaps.
References & Sources
- In The Swim. “How to Measure for a Custom Safety Pool Cover.” Provides the official A-B triangulation measurement process.
- Pool Guard USA. “How to Measure for a Pool Cover.” Details the two-stake setup and obstruction calculation rules.
- VEVOR. “VEVOR Pool Safety Cover 20×40 ft Rectangular In-Ground.” Product specs and weight for a typical 20×40 safety cover.
- River Pools. “Pool Cover Sizing, Types, and Prices.” Pricing data for custom vs. standard winter covers.
- INYO Pools. “How to Select the Correct Sized In-Ground Winter Pool Cover.” Explains the water-level drop and obstruction calculation rules.