Step 1 — Walk the perimeter and identify the spine lines
Before you touch the calculator, walk the bed once with no tape in hand. You're looking for the straight architectural lines that frame it — the house wall, a fence, a sidewalk, the edge of a driveway. These become the long edges of your rectangles.
Step 2 — Sketch it on whatever's handy
- Grab graph paper, the back of a seed envelope, or your phone's notes app — anything that lets you draw to scale.
- Mark each rectangle with a letter (A, B, C) and write the length × width inside.
- Where two rectangles share an edge, draw a heavy line so you don't double-count.
- Check for overlap at corners: if rectangle B starts inside rectangle A, shrink B until it begins exactly at A's edge.
- Annotate any subtraction zones — stepping stones, a birdbath base, an open mulch-free trunk clearance — to subtract after summing.
Step 3 — Enter each rectangle and sum
Step 4 — Account for curves and subtractions
Curves are the only place this method gets noticeably approximate. The two ways to handle them:
- Stair-step: trace the curve with 3–5 short rectangles that follow it. Accurate to within 5% if the rectangles overlap the curve evenly.
- Ellipse formula: for a full oval section, use length × width × 0.785 as the area and enter it as a single zone with width = 1 ft, length = the computed area.
- Half-circle: for a rounded end-cap, use π × radius² ÷ 2, then enter as one zone the same way.
Subtractions go in last. Sum the additive zones, then deduct any large hardscape (over 10 sq ft) before applying depth.
Step 5 — Order with a 10% settling buffer
| Zone count | Typical bed | Accuracy vs survey | Time to measure |
|---|---|---|---|
| 1 rectangle | Simple straight border | 98–100% | 2 min |
| 2 rectangles | L-shape wraparound | 96–98% | 4 min |
| 3 rectangles | U-shape patio surround | 95–97% | 6 min |
| 4+ rectangles | Multi-zone foundation bed | 93–95% | 10 min |
| With curves stair-stepped | Curved peninsula border | 90–93% | 12 min |