Window Seat Cushions for Bay Windows: How to Measure and Order

Quick answer: To order window seat cushions for a bay, measure each straight run of the seat on its own: back edge, front edge and depth, in centimetres. Most Victorian canted bays have sides at about 45°, so the usual set is a rectangle or trapezoid for the centre plus two trapezoids. Out-of-square bays need a paper template.

A bay window seat is the best spot in many British terraces, and the hardest to fit a cushion to. The walls are rarely square and the glass lets in sun and cold. Here is how to measure, choose a layout and pick a fabric that copes.

What shape is a Victorian or Edwardian bay window?

Most period bays in the UK are canted: a straight front with two angled sides. According to The Victorian Emporium, most bays have side splays at 45° to the main walls of the house.

The same source notes that poor bricklaying can leave a splay at 43°, 46° or 47°. That small drift matters when two pieces of foam must meet in a neat corner. You will also find square bays, curved bow windows and the shallow bays common on 1930s semis.

Should you get one long cushion or three angled sections?

For a seat that follows the angles of a canted bay, three sections fit better and are easier to live with. One long cushion only works when the seat itself is a straight run.

A trapezoid is a four-sided shape with two parallel edges of different lengths. In a 45° bay, the two side cushions are trapezoids, and the centre cushion is usually a trapezoid too, with a shorter front edge than back edge. The angled ends meet in a neat mitre at each corner.

If your seat runs straight across the front of the bay, or sits in a flat alcove, a single long made-to-measure window seat cushion is simpler and looks cleaner.

A seat that turns a 90° corner can take a one-piece L-shaped cushion, which avoids a gap where two cushions meet.

How do you measure each section and the angles?

Measure every section separately, along the back edge, the front edge and the depth. Never copy one side's numbers to the other, because the two splays often differ.

1. Measure the back edge of the centre section, where the seat meets the window wall. Write it down in centimetres.
2. Measure the front edge of the same section, along the edge of the seat board.
3. Measure the depth, from the back wall to the front edge, at both ends and in the middle. Use the smallest figure.
4. Repeat for the left side and the right side. Note which end of each side meets the centre.
5. Check the angle at each corner with a sliding bevel or angle finder. If you do not have one, a template is safer.

Here is a rough guide for a true 45° bay. At each angled end, the front edge of the cushion is shorter than the back edge by about 0.41 times the seat depth. Say your seat is 45 cm deep and the centre back edge is 120 cm. Each angled end loses about 18.5 cm at the front, so the front edge is close to 83 cm.

Treat that as a check, not a cutting list. For more tips, see our guide on how to measure cushions in cm.

When do you need a paper template?

Make a template when your corners are not true angles, the walls bow, or the seat curves. Old brick bays are often a few degrees out, and a template captures that exactly.

1. Tape sheets of brown paper or lining paper over the whole seat, overlapping the joins.
2. Press the paper into each corner and crease it firmly along the walls and the front edge.
3. Draw along the creases with a marker, then mark where each cushion should split.
4. Label every piece "top", "front", "left" or "right".
5. Cut the pieces out and lay them back on the seat to check the fit.

Our custom shaped cushions page explains how to send it. For simple angled pieces with good measurements, you can order a custom trapezoid cushion directly instead.

How thick should a window seat cushion be?

For most window seats, 76 mm to 102 mm (3 to 4 in) of foam is enough. Go thicker only if the seat board is low and the window opens well above it.

Check that the cushion will not stop a sash or casement opening, and that your feet still reach the floor. Our standard range is 76 mm to 127 mm, up to 203 mm in the builder, and thinner on request.

For daily sitting, choose high density foam (our all-purpose foam) or firm luxury foam. Keep super soft foam for back cushions only. Foam for UK orders is made and cut in the UK.

Which fabrics resist sun fade through glass?

Choose a solution-dyed fabric such as Sunbrella. Window glass filters some of the sun, but not all of the UV that fades colour.

The Irish Skin Foundation explains that UVB does not pass through window glass, but UVA may get through, depending on the type of glass. UVA is the longer-wave ultraviolet light that can fade dyes. The Met Office, quoting Cancer Research UK, says the sun can be strong enough to burn in the UK from around April to September.

Sunbrella says its fabrics are 100% solution-dyed acrylic. The colour goes into the fibre before it is spun, so it runs right through. When comparing other fabrics, look for a colour fastness to light rating, which in the UK is tested under BS EN ISO 105-B02 using a lamp that copies daylight.

We stock Sunbrella and Warwick, with 10,000+ colours overall. Order swatches and check them in your own light.

How do you stop condensation and damp on a window seat?

Keep air moving around the cushion and the glass warm. Condensation forms when warm, moist air meets cold surfaces such as windows, and a window seat sits right against the glass.

Rother District Council says a family of four puts 30 to 40 litres of water into the air each week just by breathing. Cooking and bathing add about 20 litres more, and drying clothes indoors adds 10 to 15 litres.

Leeds City Council suggests keeping your home between 18°C and 21°C and wiping condensation off windows.

On the seat itself, lift the cushions now and then on cold mornings so the board can dry. Leave a small gap between the cushion back and the glass where you can. Sunbrella says its fibres resist mould and mildew, and for a very damp bay, quick-drain open-cell foam lets water run straight through.

If the bay still has single glazing, Historic England says secondary glazing can cut heat loss by nearly 60%. A warmer window gives condensation less chance to form.

How long do made-to-measure window seat cushions take?

Standard shapes, such as rectangles and trapezoids from your measurements, take 2 to 3 weeks. Template shapes take 5 to 6 weeks, because each piece is cut to your paper pattern.

We cut within 6 mm (maximum ±2.5 cm) of your measurements, and if we miss that, we remake the cushion free. Made-to-measure cushions cannot be returned for wrong measurements, so check twice. UK delivery is free, with import duty and VAT already paid, so there is nothing to pay on arrival. Browse the full range of window seat cushions to get started.

Frequently asked questions

What angle are Victorian bay windows?

Most Victorian canted bays have side splays at about 45° to the main wall. Older brickwork can leave them a few degrees out, at 43° to 47°. Measure each corner, or make a paper template if you are unsure.

Can I order just the covers for my window seat?

Yes. You can order a complete cushion, a cover only or foam only. Measure the old foam carefully first.

Do window seat cushions need non-slip backing?

We do not add rubber or non-slip backing. It voids the warranty and stops the cover being machine washable. Fabric ties are standard, and Velcro is available on request.

What is the best foam for a window seat?

High density foam suits most window seats. Firm luxury foam feels firmer. For a damp bay, quick-drain open-cell foam lets moisture pass through.

How long does a template-cut bay cushion take?

Template shapes take 5 to 6 weeks. Standard shapes from measurements take 2 to 3 weeks.

Sources

• The Victorian Emporium: How to create an authentic Victorian bay window
• Irish Skin Foundation: Clouds, water, glass: surprising things that solar UV can pass through
• Met Office: UV levels
• Sunbrella: The Sunbrella fabric difference
• BSI: BS EN ISO 105-B02 Colour fastness to artificial light
• Rother District Council: Damp, mould and condensation
• Leeds City Council: Damp, mould and condensation
• Historic England: Traditional Windows, their care, repair and upgrading