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Thread Length Calculator

Work out how much thread a saddle stitched seam needs, in both directions, with the waste and the needle loading allowed for.

Leather Craft
Running out of thread two thirds of the way along a seam is one of the small miseries of leatherwork, and it always happens on the seam where the thread is a special order. This calculator takes a seam length, the thread consumption factor and the number of seams and returns the length to cut, in both directions, with an allowance for the tail and the loading. It also tells you how much thread a whole project consumes, which is what you need to buy a spool.

At a glance

What you enter
Seam length, Number of seams, Thread consumption factor, Tail allowance per needle, Waste allowance, Thread on one spool
What you get
Thread to cut for one seam, Thread per seam, Thread for all the seams, Thread for all the seams, Thread for all the seams, Thread length as a multiple of the seam, Thread for both directions, before the tails, Allowance for the two tails, Thread allowed for waste and fraying, Spools to buy, Spools for ten projects, Share of a spool this project uses, Stitches at a 4 mm spacing, Long length, watch for tangling, Thread to have on hand
Formula shown
Yes, every step, right below your results.
Cost and privacy
Free, no account needed, and nothing you type is stored.

See worked examples ↓ Read the FAQ ↓ Share this calculator ↓

One seam, measured along the stitching line.

The total thread a seam consumes as a multiple of its length, covering BOTH directions. Seven to eight is the usual figure for hand sewing, being three and a half to four times the seam for each of the two passes.

Thread left hanging past each needle plus the last few stitches. A foot at each end is a practical figure.

Fraying, a cut out knot and the tail after backstitching. Ten percent for waxed thread, fifteen for a soft thread.

Optional, for the spool count. A common spool is 25 metres.

Reset

Results are estimates for general information only, always verify with a professional or official source before acting on them.

How to use the Thread Length Calculator

  1. 1 Enter Seam length (mm), One seam, measured along the stitching line.
  2. 2 Enter Number of seams
  3. 3 Enter Thread consumption factor, The total thread a seam consumes as a multiple of its length, covering BOTH directions. Seven to eight is the usual figure for hand sewing, being three and a half to four times the seam for each of the two passes.
  4. 4 Enter Tail allowance per needle (mm), Thread left hanging past each needle plus the last few stitches. A foot at each end is a practical figure.
  5. 5 Enter Waste allowance (%), Fraying, a cut out knot and the tail after backstitching. Ten percent for waxed thread, fifteen for a soft thread.
  6. 6 Enter Thread on one spool (m), Optional, for the spool count. A common spool is 25 metres.
  7. 7 Press Calculate. You get Thread to cut for one seam, Thread per seam, Thread for all the seams, Thread for all the seams, Thread for all the seams, Thread length as a multiple of the seam, Thread for both directions, before the tails, Allowance for the two tails, Thread allowed for waste and fraying, Spools to buy, Spools for ten projects, Share of a spool this project uses, Stitches at a 4 mm spacing, Long length, watch for tangling, Thread to have on hand, and every intermediate step stays visible under "How is this calculated?"
  8. 8 Change one number at a time to see what moves the result, that is usually the most useful thing a calculator can tell you.

Worked examples

Real numbers run through this calculator. Open any example to load it into the form above and change it.

Four 300 mm seams

A common bifold wallet: two seams down the sides and two across the card slots.

You enter

Seam length
300 mm
Number of seams
4
Thread consumption factor
7.60
Tail allowance per needle
300 mm
Waste allowance
10%
Thread on one spool
25 m

You get

Thread to cut for one seam
3,168.00 mm
Thread per seam
3.17 m
Thread for all the seams
12,672.00 mm
Thread for all the seams
12.67 m
Thread for all the seams
13.86 yd
Thread length as a multiple of the seam
10.56
Thread for both directions, before the tails
2,280.00 mm
Allowance for the two tails
600.00 mm
Thread allowed for waste and fraying
1,152.00 mm
Spools to buy
1
Spools for ten projects
6
Share of a spool this project uses
0.51
Stitches at a 4 mm spacing
300
Long length, watch for tangling
0
Thread to have on hand
15.21 m

About 3.17 metres per seam and 12.7 metres for the project, which is half of a 25 metre spool. The total is about 10.6 times the seam once the tails and the waste are counted.

Open this example in the calculator →

A single long belt seam

One 1100 millimetre seam round a belt, where running out would be expensive.

You enter

Seam length
1,100 mm
Number of seams
1
Thread consumption factor
8
Tail allowance per needle
400 mm
Waste allowance
12%
Thread on one spool
25 m

You get

Thread to cut for one seam
10,752.00 mm
Thread per seam
10.75 m
Thread for all the seams
10,752.00 mm
Thread for all the seams
10.75 m
Thread for all the seams
11.76 yd
Thread length as a multiple of the seam
9.77
Thread for both directions, before the tails
8,800.00 mm
Allowance for the two tails
800.00 mm
Thread allowed for waste and fraying
1,152.00 mm
Spools to buy
1
Spools for ten projects
5
Share of a spool this project uses
0.43
Stitches at a 4 mm spacing
275
Long length, watch for tangling
1
Thread to have on hand
12.90 m

About 10.7 metres for one seam, which is a long thread to manage and worth waxing before you start.

Open this example in the calculator →

A run of ten card holders

Two 180 millimetre seams each, on ten items at once.

You enter

Seam length
180 mm
Number of seams
20
Thread consumption factor
7
Tail allowance per needle
250 mm
Waste allowance
10%
Thread on one spool
25 m

You get

Thread to cut for one seam
1,936.00 mm
Thread per seam
1.94 m
Thread for all the seams
38,720.00 mm
Thread for all the seams
38.72 m
Thread for all the seams
42.34 yd
Thread length as a multiple of the seam
10.76
Thread for both directions, before the tails
1,260.00 mm
Allowance for the two tails
500.00 mm
Thread allowed for waste and fraying
3,520.00 mm
Spools to buy
2
Spools for ten projects
16
Share of a spool this project uses
1.55
Stitches at a 4 mm spacing
900
Long length, watch for tangling
0
Thread to have on hand
46.46 m

About 34 metres across the batch, so two 25 metre spools is the sensible purchase.

Open this example in the calculator →

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Frequently asked questions

How much thread do I need for a saddle stitched seam?
About four times the seam length for each of the two passes, so seven to eight times the seam length in total, and a little over ten times once the tails and the waste are added. A 300 millimetre seam therefore takes about 3.2 metres of thread cut as a single length. The rule of thumb is usually stated as three and a half to four times, and the part people forget is that it applies to each direction, so the total is double it.
Why is the thread cut as one length rather than two?
Because a saddle stitch needs a needle at each end of the same thread. The two needles pass through each hole in opposite directions and the threads cross, which locks each stitch and is what makes the seam hold if one stitch is cut. Two separate threads, one from each side, do not lock and the seam will loosen. This is also why the thread is measured as a single length in the answer.
How long should the tails be?
About 300 millimetres or a foot at each end, which is 600 millimetres of slack in total. The tail covers the thread that is consumed by the needle loading, the last few stitches where the thread is thinned and waxed away, and the backstitch at the end. More is not harmful because unused thread can be used for a smaller seam, whereas a seam that runs short has to be entirely unpicked and resewn.
Does waxing the thread change the length needed?
It does not change the length, but it changes how much of it you lose. Waxed linen or polyester frays far less at the ends, tangles less and holds the stitches more firmly, so a ten percent waste allowance is realistic. Unwaxed thread frays and can lose noticeably more, so allow fifteen percent. If you are using a thread conditioner rather than wax, the effect is similar and the same allowances apply.
What if my thread length is longer than my arm?
It will tangle, and managing it becomes the hard part of the seam rather than the stitching. Above about two metres, coil the thread in figure eights around two fingers and let it pay out as you work, and wax it. For a very long seam, some makers work in sections with a backstitch at each join, which is slightly weaker but far more practical than wrestling with six metres of loose thread.

More money questions answered →

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