The wrong measurement on a bay window is the most expensive mistake in shutters. Get it 5mm wrong on a splay bay and the panels won't fold flat — and a remake costs £400+. Here's exactly how we measure bay windows on every survey across Warrington and Cheshire.
Step 1: identify your bay type
Three common bay shapes, each with its own measuring trap:
- Square bay (90°). Three sections meeting at right angles — common on 1930s semis in Appleton and Grappenhall.
- Splay bay (typically 135°). The most common UK bay — Victorian and Edwardian fronts in Stockton Heath, Lymm and Knutsford.
- Curved bay. No flat sections — the frame follows a continuous arc. Shutters here need bespoke shaped panels.
Step 2: width — three readings, smallest wins
Measure each bay section at top, middle and bottom of the recess. Brickwork is never square — variations of 8–12mm are normal in a Victorian house. Always use the smallest reading. If you use the largest, the shutter won't fit; if you use the average, you'll get binding panels.
Step 3: height — three readings again
Measure left, centre and right of each section, sill to head. Old cills sag — Edwardian houses in particular. Take the shortest reading. For a tier-on-tier shutter (two independent panels stacked), note the mid-rail height too.
Step 4: recess depth — the deal-breaker
You need a minimum of 70mm recess depth to fit shutters fully inside the recess with louvres clearing the glass. Less than that and you're into face-fit territory (frame sits proud of the wall). Measure depth at the corner where the shutter hinge will sit — that's the critical point.
Step 5: bay angles (splay bays only)
A bevel gauge or angle finder app on a phone works fine. Read each internal angle — they should match within 2°. If they don't, the bay was built off-square and the shutter frame needs scribing on site. Tell your fitter on the survey, not at install.
The four numbers that decide your price
(1) Total width across all bay sections (2) Tallest section height (3) Number of sections (4) Material choice. Get those four to a fitter and you'll get a budget number inside 5 minutes. Anyone quoting without a site visit is guessing — bay windows always need measuring in person.