Weaving Warp Calculator
From finished cloth to a dressed loom — total warp length, ends and yarn weight with take-up, shrinkage and loom waste, plus a yarn-count converter and a reed denting pattern you can see.
Add yarn count for a weight estimate (optional)
How the weaving warp calculator works
Dressing a loom starts long before the first pick. You have to know how much warp to measure, how many ends to wind, how wide to sley the reed, and how much yarn to buy — and every one of those numbers depends on the others. Guess low and you run out of warp mid-project; guess high and you waste expensive yarn. This calculator walks the whole chain from the cloth you want to the warp you wind, and shows its working at each step so you can trust the total and adjust with confidence.
It runs in three parts. The Warp Planner turns finished dimensions into total warp length, the number of ends, and — if you add a yarn count — the weight to purchase. The Yarn Weight converter translates any of the world's yarn-count systems into a concrete weight. And the Reed Denting tool solves the everyday puzzle of threading a sett your reed doesn't divide evenly, drawing the exact sleying pattern.
Take-up, draw-in and shrinkage: why cloth comes up short
When warp and weft interlace, they don't lie straight — each yarn follows a wavy path over and under the other, a geometry textile engineers call crimp. The consequence is that a length of yarn produces a shorter, narrower piece of cloth than its raw measurement. Lengthwise, this loss is take-up; widthwise, it's draw-in. Because warp is held under high tension and weft is inserted more loosely, weft usually crimps more, so draw-in often exceeds take-up on the loom.
Then the cloth is washed, and it shrinks again. Wet finishing has three mechanisms: relaxation, as tensioned fibres relax back; consolidation, as hydrophilic fibres swell and pull the structure tighter; and felting, the irreversible interlocking of wool scales. The calculator lets you fold both loom take-up and wash shrinkage into a single lengthwise and widthwise percentage. If you haven't woven a test sample, these defaults get you close.
| Fibre | Typical shrinkage | Note |
|---|---|---|
| Pre-shrunk / sanforized cotton | 2–3% | Compacted at the mill |
| Raw cotton (plain weave) | 5–10% | Swells; warp shrinks more |
| Linen & hemp | 7–10% | High first-wash contraction |
| Rayon / viscose | 10–12% | Severe swelling |
| Untreated wool | 15–20%+ | Felts unless superwash |
| Silk | 1–3% | Very stable |
| Poly-cotton blend | 3–5.5% | Polyester anchors it |
Loom waste: the length you never weave
Every warp gives up some length that can never become cloth: the yarn tied to the front apron rod, the unwoven header, and the tail at the back where the heddles stop the beater short of the back beam. This loom waste is added once per warp, not per piece, and it scales with the loom. A compact rigid-heddle loom gives up only 8–15 inches; a table loom 18–24; a big floor loom 24–36 or more, because of the long span between its beams. On sectional systems it varies by tension box and beam sections. Under-budgeting loom waste is the classic way to come up short, so the calculator offers presets by loom type and adds the waste to your total automatically.
Global yarn counts and the weight you buy
Weavers plan in length but buy by weight, and the bridge is the yarn's count — and there are many systems. Indirect counts measure length per unit weight, so a higher number means a finer yarn: English cotton (Ne) counts 840-yard hanks per pound, worsted (NeW) 560-yard hanks, linen (Lea) 300-yard leas, and metric (Nm) 1,000-metre lengths per kilogram. Direct counts do the opposite, measuring weight per length, so a higher number is coarser: Tex is the grams per 1,000 metres, denier the grams per 9,000 metres. The engineering bridge between them is Ne = 590.5 ÷ Tex and Tex = denier ÷ 9. Plied yarns like 8/2 divide the base count by the plies for an effective count. The converter turns any of these into yards per pound or metres per kilogram, then into the exact weight your project needs.
Reed denting: threading a sett your reed won't divide
Your reed has a fixed number of slots per inch — its dents per inch — but the sett you want rarely matches it. When ends per inch don't divide evenly into dents per inch, you thread a repeating pattern so the density averages out. The maths is short: divide EPI by DPI for the base threads in every dent, take the remainder, and reduce the fraction of remainder over DPI by its greatest common divisor. The reduced fraction tells you how many extra threads to spread over how many dents.
For example, 14 ends per inch in a 10-dent reed gives a base of 1 thread per dent with a remainder of 4; the fraction 4/10 reduces to 2/5, meaning 2 out of every 5 dents take a second thread. The calculator computes this and draws the pattern dent by dent, so you can sley straight from the screen instead of hunting through a printed chart.
A note on method. These are planning figures built from standard textile formulas and published shrinkage and loom-waste ranges. Real cloth varies with fibre, yarn, tension, weave structure and finishing, so the surest path is to weave, wash and measure a sample and feed your own take-up and shrinkage back into the tool. Every calculation here is shown step by step so you can adjust it to your loom and your yarn.
FAQ
How do you calculate warp length for weaving?
Add take-up and shrinkage (often 10–15%) to each piece's finished length for the woven length, multiply by the number of pieces, then add unwoven warp — fringe between pieces plus loom waste. Divide by 36 for yards. The Warp Planner does this and shows every step.
How much loom waste should I allow?
It depends on the loom: rigid heddle 8–15", table loom 18–24", floor loom 24–36"+. Add it once per warp, not per piece. The calculator has presets by loom type.
How do I convert yarn count to yards per pound?
Multiply an indirect count by its hank length: Ne × 840, NeW × 560, Lea × 300; Nm × 1,000 gives metres per kilogram. For plied yarn like 8/2, divide the base count by the plies first. Tex and denier are direct — convert via Ne = 590.5 / Tex. The converter handles all of them.
What is reed denting or sleying?
It's how you thread ends through the reed's dents. When EPI doesn't divide evenly into DPI, divide for the base per dent, take the remainder, and reduce remainder/DPI by its GCD. For 14 EPI in a 10-dent reed: base 1, and 2 of every 5 dents get a second thread. The tool draws the pattern.
What is take-up and draw-in?
As yarns interlace they follow a wavy path, so cloth ends up shorter (take-up) and narrower (draw-in) than the yarn laid in. Weft draw-in is usually larger. Add take-up to length and draw-in to width before figuring ends and yarn — the calculator does this automatically.
How much does fabric shrink when washed?
Rough guides: pre-shrunk cotton 2–3%, raw cotton 5–10%, linen/hemp 7–10%, rayon 10–12%, untreated wool 15–20%+, silk 1–3%. Weaving and washing a sample is most accurate, but these defaults let you plan ahead.