# How the FabricMeasure calculators work

Every figure printed on this site is produced by the same function the calculator runs.
The calculations return their working alongside the result, the pages render it, and a test
pins each documented worked example to that function’s output — so an explanation cannot
drift away from the tool it explains.

Source: https://fabricmeasure.com/methodology/
Last updated: 2026-08-20
Published by FabricMeasure.

---

## The rules that apply everywhere

**Rounding is always up, never to nearest, at every step of every calculation.** The error is
deliberately asymmetric: buying 0.4 m too much costs a little, while buying 0.2 m too little
means re-ordering from a dye lot that may have sold out, which is not recoverable.

**Side hems are charged to the two outer edges only.** Interior edges between joined widths are
seams, not hems. Charging a side hem to every width — the intuitive version — over-counts by
roughly one hem per join.

**One extra pattern repeat is added for placement**, so the first cut can start where you want it
in the design. Many published calculators do not add this.

**Cutting plans are a heuristic, not an optimal layout.** First-fit-decreasing shelf packing will
not beat a cutter with the cloth in front of them, but it never under-orders — the same direction
as every rounding rule above.

**Conversions are exact, not approximate.** One yard is 0.9144 m and one inch 25.4 mm, by the
International Yard and Pound Agreement of 1959 — [Refinement of Values for the Yard and the
Pound](https://www.nist.gov/document/frn-59-5442-1959pdf), Federal Register, 1 July 1959. The
same notice fixes the avoirdupois pound at 0.45359237 kg, so the ounce is 28.349523125 g. Mass
per unit area is measured by the method in [ISO 3801:1977](https://www.iso.org/standard/9335.html),
confirmed as current in 2023. Nominal roll widths are NOT exact: a roll sold as 54 in measures
137.16 cm while one sold as 140 cm really is 140.

## Where this differs from common trade practice

| Rule here | What many calculators do |
| --- | --- |
| Round up at every step | Round to nearest, or round once at the end |
| Add one repeat for pattern placement | Add none |
| Take side hems and seams off the roll before solving the widths | Treat the full roll width as usable |
| Add returns and centre overlap to the span | Use the bare track width |
| Solve each curtain separately, in whole widths | Round the total, allowing half widths a side |
| Refuse to answer when an input makes the maths meaningless | Print a number anyway |

**Checked on 21 August 2026 against three published calculators in two markets** — two UK
curtain suppliers and a US drapery supplier, eleven comparisons in all. Every difference is one
of the above, and every one of them asks you to buy more cloth rather than less. The cut length
agreed to the centimetre on every case and every source. The deduction of side hems from the
roll, which neither UK consumer calculator makes, is made by the US drapery supplier exactly as
it is here: it is a trade convention rather than a national one. The full case-by-case
comparison is on the methodology page.

## Default heading allowances

| Heading | Heading allowance | Hem | Minimum fullness | Recommended |
| --- | --- | --- | --- | --- |
| Pencil pleat | 15 cm | 20 cm | 2x | 2.5x |
| Pinch pleat | 20 cm | 20 cm | 2x | 2.2x |
| Eyelet | 25 cm | 20 cm | 1.8x | 2x |
| Tab top | 12 cm | 20 cm | 1.5x | 1.8x |
| Wave | 15 cm | 20 cm | 2x | 2.2x |

## Nominal roll widths, and what they measure

| Sold as | Actually measures | Typically |
| --- | --- | --- |
| 36 in / 90 cm | 91.4 cm | Quilting cottons, older dress fabrics |
| 44 in / 112 cm | 111.8 cm | Standard quilting cotton |
| 45 in / 115 cm | 114.3 cm | Dressmaking cottons, commercial patterns |
| 54 in / 137 cm | 137.2 cm | Upholstery, curtain lining |
| 55 in / 140 cm | 139.7 cm | Curtain and furnishing fabrics |
| 60 in / 150 cm | 152.4 cm | Knits, wide dress fabrics |
| 108 in / 274 cm | 274.3 cm | Wide-back quilting fabric |

## Common seam allowances

| Allowance | In millimetres | Used for |
| --- | --- | --- |
| ¼ in | 6.35 mm | Patchwork and quilting, where every seam is pieced. |
| ⅜ in | 9.52 mm | Overlocked knits and fine seams. |
| 1 cm | 10 mm | European patterns, bag making, home furnishing. |
| ½ in | 12.7 mm | General home sewing, US patterns for straight seams. |
| 1.5 cm | 15 mm | The metric dressmaking standard. |
| ⅝ in | 15.88 mm | The imperial dressmaking standard. Not the same as 1.5 cm. |
| 2 cm | 20 mm | Curtains, loose covers, anything that may be let out. |

## Cutting increments

What a shop will actually cut to. The result is rounded up to this, never down.

| Rule | Increment |
| --- | --- |
| Nearest 10 cm | The usual cut for roll goods anywhere fabric is sold by the metre. |
| Nearest half metre | Common for upholstery weights and wholesale bolts. |
| Whole metres | Some mills and online sellers cut only in whole metres. |
| Nearest ⅛ yard | The standard cut at a US quilting counter. |
| Nearest ¼ yard | Common for general dress and home-furnishing fabrics in the US. |
| Exact requirement | No rounding — useful when you are cutting from fabric you already own. |

## Half-drop repeats

Where a design steps down half a repeat between adjacent widths instead of lining up straight
across, set the match beside the pattern repeat: the curtain, blind and pattern repeat
calculators cost it. The cut length is unchanged and the job takes **half a repeat more in
total** — once, not once per width. A workroom cuts every odd width consecutively, shifts the
pattern a single time, then cuts the even ones, so the shift is paid for once however wide the
window. The obvious rule, half a repeat for every width after the first, over-orders.

A second workroom method cuts alternate widths A and B with each cut rounded to a half repeat,
and needs less cloth on almost every job. The figure here uses the first method, because the
quantity has to be safe for whichever method the person cutting uses — the calculators report
what the other one would save so you can ask before ordering less.

## Railroading

Turn the cloth 90° and the roll's WIDTH covers the drop while its length runs across the window:
no vertical joins at any width, and a curtain takes its own flat width plus two side hems. The
deepest drop you can railroad is the fabric width less the heading, the hem and anything
unusable. A 300 cm track at 2x on a 300 cm roll takes 10.2 m vertically and 6.32 m railroaded.
A vertical pattern repeat cannot be matched once the cloth is turned, and the lining is still
costed running vertically. If the roll cannot cover the drop the calculator refuses rather than
falling back to a vertical layout.

## Selvedge and unusable width

A roll sold as 140 cm does not always give 140 cm you can cut: the selvedge is usually trimmed,
a flaw can run down an edge, a washable natural can shrink in a pre-wash. The curtain and blind
calculators take an **unusable width** and deduct it from EVERY width — every width has two
selvedges, unlike the side hems, which are turned on the two outer edges of a pair however many
widths it takes. Default zero, so the roll is taken as sold unless you say otherwise.

## Roman blind folds

The drop divides into an **odd** number of equal sections. The bottom section hangs flat and
never folds; every pair above it folds over a rod, so a fold is two sections deep and the flat
bottom is half a fold — which is why a blind planned with equal folds has its bottom edge
overhanging the first pleat. Rod pockets = (sections − 1) ÷ 2, and each rod sits at an odd
multiple of the section depth measured up from the bottom edge.

## What is deliberately not modelled

- **Shrinkage**, beyond any allowance you enter yourself.

## Every tool, with its method version

| Tool | Method | Updated | Markdown |
| --- | --- | --- | --- |
| [Curtain fabric calculator](https://fabricmeasure.com/calculators/curtain-fabric-calculator/) | v4 | 2026-08-20 | https://fabricmeasure.com/calculators/curtain-fabric-calculator.md |
| [Yards to metres](https://fabricmeasure.com/conversions/yards-to-metres/) | v1 | 2026-08-20 | https://fabricmeasure.com/conversions/yards-to-metres.md |
| [Metres to yards](https://fabricmeasure.com/conversions/metres-to-yards/) | v1 | 2026-08-20 | https://fabricmeasure.com/conversions/metres-to-yards.md |
| [Fabric width converter](https://fabricmeasure.com/conversions/fabric-width-converter/) | v1 | 2026-08-20 | https://fabricmeasure.com/conversions/fabric-width-converter.md |
| [Curtain fullness calculator](https://fabricmeasure.com/calculators/curtain-fullness-calculator/) | v1 | 2026-08-20 | https://fabricmeasure.com/calculators/curtain-fullness-calculator.md |
| [Pattern repeat calculator](https://fabricmeasure.com/calculators/pattern-repeat-calculator/) | v2 | 2026-08-20 | https://fabricmeasure.com/calculators/pattern-repeat-calculator.md |
| [Fabric yardage calculator](https://fabricmeasure.com/calculators/fabric-yardage-calculator/) | v1 | 2026-08-20 | https://fabricmeasure.com/calculators/fabric-yardage-calculator.md |
| [Cushion fabric calculator](https://fabricmeasure.com/calculators/cushion-fabric-calculator/) | v1 | 2026-08-20 | https://fabricmeasure.com/calculators/cushion-fabric-calculator.md |
| [Upholstery fabric calculator](https://fabricmeasure.com/calculators/upholstery-fabric-calculator/) | v1 | 2026-08-20 | https://fabricmeasure.com/calculators/upholstery-fabric-calculator.md |
| [Seam allowance calculator](https://fabricmeasure.com/calculators/seam-allowance-calculator/) | v1 | 2026-08-20 | https://fabricmeasure.com/calculators/seam-allowance-calculator.md |
| [Fabric GSM calculator](https://fabricmeasure.com/conversions/fabric-gsm-calculator/) | v1 | 2026-08-20 | https://fabricmeasure.com/conversions/fabric-gsm-calculator.md |
| [Standard curtain sizes](https://fabricmeasure.com/charts/standard-curtain-sizes/) | v1 | 2026-08-20 | https://fabricmeasure.com/charts/standard-curtain-sizes.md |
| [Fabric width chart](https://fabricmeasure.com/charts/fabric-width-chart/) | v1 | 2026-08-20 | https://fabricmeasure.com/charts/fabric-width-chart.md |
| [Seam allowance chart](https://fabricmeasure.com/charts/seam-allowance-chart/) | v1 | 2026-08-20 | https://fabricmeasure.com/charts/seam-allowance-chart.md |
| [Fabric weight chart](https://fabricmeasure.com/charts/fabric-weight-chart/) | v1 | 2026-08-20 | https://fabricmeasure.com/charts/fabric-weight-chart.md |
| [Upholstery yardage chart](https://fabricmeasure.com/charts/upholstery-yardage-chart/) | v1 | 2026-08-21 | https://fabricmeasure.com/charts/upholstery-yardage-chart.md |
| [Curtain heading types](https://fabricmeasure.com/charts/curtain-heading-types/) | v1 | 2026-08-20 | https://fabricmeasure.com/charts/curtain-heading-types.md |
| [Curtain lining calculator](https://fabricmeasure.com/calculators/curtain-lining-calculator/) | v1 | 2026-08-20 | https://fabricmeasure.com/calculators/curtain-lining-calculator.md |
| [Roman blind fabric calculator](https://fabricmeasure.com/calculators/roman-blind-fabric-calculator/) | v4 | 2026-08-20 | https://fabricmeasure.com/calculators/roman-blind-fabric-calculator.md |
| [Curtain heading tape calculator](https://fabricmeasure.com/calculators/curtain-heading-tape-calculator/) | v1 | 2026-08-20 | https://fabricmeasure.com/calculators/curtain-heading-tape-calculator.md |
| [Tablecloth fabric calculator](https://fabricmeasure.com/calculators/tablecloth-fabric-calculator/) | v1 | 2026-08-20 | https://fabricmeasure.com/calculators/tablecloth-fabric-calculator.md |
| [Bias binding calculator](https://fabricmeasure.com/calculators/bias-binding-calculator/) | v1 | 2026-08-20 | https://fabricmeasure.com/calculators/bias-binding-calculator.md |

## A caution

These figures are a guide to help you plan a purchase, not a professional measuring service.
Check them against your own job before cutting anything or placing an order.
Terms: https://fabricmeasure.com/terms/