Door Threshold and Window Sill Calculator

How the calculation works

Door thresholds and window sills: how to work out the metres to order

This calculator estimates how many linear metres of stone you need to order for the doors and windows in a project. It takes the width of each opening, adds a side overlap at each end, multiplies the result by the number of pieces, and then adds a waste allowance. The output is a single total in metres that you can pass to a supplier or a stonemason.

The calculation is deliberately simple, because the order quantity depends on only a few measurable inputs. The calculator does not size the depth of the stone, check its structural suitability or choose the fixing method. Those depend on the wall thickness, the finish and the project specification, and they are covered later in this section.

What a threshold and a sill do

A threshold is the piece fitted at floor level across the base of a door opening. It marks the transition between two rooms, or between inside and outside, and on external doors it also helps keep out draughts and water. A window sill, sometimes called a window cill, is the equivalent piece at the base of a window. On external walls it usually projects beyond the face of the wall, so that rainwater runs off the wall rather than soaking into the masonry below the frame.

In both cases the piece has to be longer than the opening. It rests on the masonry at each end, and that bearing holds it in place, together with the mortar bed, the fixings and the adjacent wall. This is why the calculator always adds the side overlap to the opening width.

The formula, step by step

For each piece, the length to order is the opening width plus twice the side overlap. The total is then the number of thresholds multiplied by their length, plus the number of sills multiplied by their length. That sum is increased by the waste allowance, expressed as a percentage of the total, and not applied to each piece separately.

Take a first-floor flat in Edinburgh with one 0.80 m kitchen door that needs a threshold and two 1.50 m windows with sills. Using a 3 cm overlap on each side, each threshold is 0.80 + 0.06 = 0.86 m and each sill is 1.50 + 0.06 = 1.56 m. The two sills come to 3.12 m, and the raw total is 0.86 + 3.12 = 3.98 m. Because the sills are cut to fit an irregular bay, a 10% allowance is used: 3.98 × 1.10 = 4.378 m, or 4.38 m to order.

The same method gives the figure in the introduction. Three 0.96 m thresholds and four 1.26 m sills make 7.92 m, and a 5% allowance brings the order to about 8.3 m.

Choosing the side overlap

The side overlap is the length of stone that sits in the masonry at each end of the opening. The overlap is set by the design rather than by the calculator. The architect or stonemason specifies it, and the value entered should match that specification. The worked examples on this page use 3 cm per side throughout, so the arithmetic is easy to check.

A sill that projects outside and faces wind and rain often calls for a firmer bearing than a threshold bedded inside. If thresholds and sills need different overlaps, work out the lengths of each type separately and then add the totals.

Check the overlap against the depth of the reveal and the specification from the architect or the stonemason. A shallow reveal may need a different fixing approach, and the calculator cannot detect that from the width alone.

Measuring the opening

Measure the width at the narrowest point between the jambs (the vertical sides of a door frame) or the reveals (the inside faces of a window opening). Take several readings, because walls are rarely perfectly straight, and use the smallest reliable figure.

On a new build where plaster has not yet been applied, the measured opening will be larger than the finished one. Deduct the thickness of the plaster on each side, and confirm that figure on site by measuring to the planned finished face. In refurbishment, measure the existing opening directly and check that the surfaces the stone will bear on are flat and level. A piece cut to a dimension taken from an uneven surface will not sit well, whatever the calculator returns.

Check the height of a threshold after the floor finish is laid, not before. Screed, tiles and timber all change the level at which the threshold sits, and the finished level decides whether the piece will sit flush with the floor.

Choosing the material

Granite is one of the most widely specified natural stones for external thresholds and sills. It is hard, resists wear and suits exposed locations. It is available in polished and flamed finishes. A flamed or bush-hammered surface is textured and less slippery underfoot, which is why it suits external thresholds. A polished finish looks sharper indoors but can become slippery when wet.

Portland stone is a limestone quarried on the Isle of Portland in Dorset. It is a traditional choice for sills on period and heritage buildings, and it matches many older facades. It is more porous than granite, so it needs careful detailing and, where it is exposed, protection from prolonged wetting. Marble gives an elegant finish for internal thresholds, but it is softer and stains more easily, so it is less suitable for external use.

Engineered quartz, a composite of crushed quartz bound with resin, is an option for internal thresholds and sills in modern interiors. Cast stone and reconstituted stone are made from cement-based or resin-bound aggregates and are produced to standard profiles. Aluminium thresholds are used where a low profile and light weight help, particularly in refurbishment.

The choice of material changes the detail but not the length calculation, as long as each length is measured in the same way. It can change the waste you should expect, because some stones break more easily in transport and cutting. For the dimensional characteristics of natural stone units, the relevant test method is BS EN 13373, which is the British adoption of the European standard. Use it as a reference when a supplier quotes tolerances, and ask for those tolerances in writing.

Waste allowance

The waste allowance covers losses that are not visible at the design stage. Pieces can break during transport, a cut can come out slightly oblique, and a measurement taken before plastering can differ from the finished opening. The right percentage depends on how much cutting and handling risk the job carries. Simple openings with straight cuts need less than irregular openings, bay windows, curved profiles or several mitred cuts. The figure is a judgement for the person ordering, so agree it with the stonemason or supplier rather than treating any fixed percentage as a rule.

The allowance is not a substitute for careful measurement. A large allowance is an expensive way to cover a poor survey. If you order in standard lengths, ask the supplier whether offcuts can be reused for smaller pieces, because that reduces the net cost of the waste.

Available lengths and stock

Stone is quarried and cut from blocks, so a supplier can only produce pieces up to a certain length. For long openings, confirm before you fix the dimensions in the design which maximum length the supplier can cut and deliver in one piece. A long single piece is harder to cut, transport and handle, and may need additional support. A stonemason can advise whether the piece should be made in two parts with a joint, and where that joint should sit.

The calculator returns metres, not pieces. The supplier will convert the order into the lengths it can cut, so ask how many pieces that involves and whether the waste allowance is already reflected in the quoted price.

Common mistakes

Confusing the opening with the piece size is the most frequent error. The opening is the gap in the wall, and the piece must always be longer than it.

Forgetting the external projection of a sill is the second. A sill is designed to project from the wall face so that it sheds water. The calculator deals with length only. The projection and the depth inside the reveal depend on the wall build-up and the design.

Leaving out the waste allowance is a third. Even accurate measurements can lead to breakage or to a cut that does not sit square.

Ordering before the openings are finished is another. A survey taken from bare brick or unplastered blockwork gives different widths from the finished opening.

Ignoring the drip groove is a fifth. A sill normally has a drip groove, also called a throating, on its underside. It stops rainwater running back along the stone to the wall. Without it, water runs into the joint with the wall, and the sill loses much of its purpose.

What the calculator does not cover

The calculator does not check structural suitability. Sills and thresholds are not lintels: they do not carry the load above the opening, which is the job of the lintel above the frame. Do not use a sill as a support.

Historic stone salvaged for reuse is often irregular and fragile, so measure each piece individually rather than relying on a standard length. Special profiles, curved edges, stair nosings and integrated weatherings also fall outside this calculation.

Sourcing in the UK

Local stonemasons and specialist stone suppliers can often produce a bespoke threshold or sill to a drawing. Builders' merchants and large DIY retailers usually stock standard sizes and cut lengths. Compare quotes on the same basis, meaning the same material, finish, edge detail and waste allowance, because a lower unit price can be offset by more waste or a different finish. Ask for written confirmation of the dimensions and the lead time before placing the order. Lead times for natural stone are often longer than for manufactured products.

When to consult a professional

Speak to a qualified stonemason or building contractor before ordering when the job involves special finishes, integrated drip details, curved or moulded profiles, or the reuse of historic stone. Works to a listed building in England or Wales can require listed building consent under the Planning (Listed Buildings and Conservation Areas) Act 1990, and replacement stone should match the original in appearance and detail. For long openings, the stone may need extra support. A professional can also confirm that the stone, the mortar and any water-repellent treatment are compatible with the location and the wall.