As a spinner, I use wraps per inch (wpi) often. If I do a good job of measuring wpi, then the square of wpi is the grist in yards per pound. And the twist factor times times wpi equals required twist in tpi.
It works. Mostly it is within 10% of the actual grist, and with corrections for fiber and construction, wpi can measure grist to within 2%, which is better than a yarn balance.
To measure wpi so that it indicates grist, you need a wpi gauge, you need to pack to refusal, and you need to practice. You need to take various kinds of yarns with known grists, and practice doing wpi, until you get the correct grist, time after time. It is a skill.
So, when a knitter says a yarn is 18 wpi, I figure the grist is 324 ypp (e.g., 18x18) -- that is like rope. Then, she gives the yarn band info of 175 yard per 2 oz. -- or 1,400 yards per pound. That is sock or fingering yarn. When I measure the wpi of that yarn, I get 37 wpi. That tells me, and other spinners, the grist.
Packing the yarn to refusal gives a definite measure that can be made anywhere by anyone. Packing the yarn loosely so that a 1,400 ypp yarn so that it yields a wpi of 18 is not a repeatable measure of -- anything. This approach is as much a measure of twist as of grist. With this approach, high twist yarns will seem to have much lower grist than yarns with the same grist, but less twist - and vice versa.
If you pack to refusal, then worsted and woolen yarns of the same grist will have the same wpi. If you use the loose knitter's wpi, then worsted and woolen yarns of the same grist will have very different wpi. If you pack to refusal then yarns of the same grist but different twist will have the same wpi. If you use the loose knitter's wpi, then yarns that are the same grist, but which have different twist will have different wpi. This is silly. What really counts to a knitter is the length and weight of the yarn. These should be described by the wpi. Twist of the yarn is less important.
In my world wpi and grist are related thusly:
wpi grist (ypp) spin count notes
22 484 Aran Yarn (traditionally was 10-ply of 10 count singles)
24 1
26 676
28 840 Worsted Yarn (traditionally was 6-ply of 10 count singles)
30 900
32 1,000 Gansey Yarn ( 5-ply of 10 count singles)
33 1,100 DK weight yarns
34 1,120 2
38 1,443 Common grist for commercial sock yarns e.g., Wooly West
40 1,650 3 My 6-strand sock yarn
42 1,800 Single cut woolen singles
44 1935 Fingering Yarn
48 2,303 Jumper Weight/ Spindrift weight 2-ply
53 2,800 5
60 3,600 2-cut woolen singles @ 9 tpi
60 3,733 7 Traditional 6-ply sock yarn from 40 count singles
64 4,100 Modern lace weight
75 5,600 10s Singles for warp/ 9-10 tpi & woolen singles for weft @ 12 tpi
80 6,700 13 Traditional 3-ply Shetland lace plied up from 40 count singles
105 11,200 20s Worsted singles that I use for my sock yarn @ 14 tpi
120 14,400 8- cut woolen singles / 18 tpi
130 16,800 30s Worsted singles @ 17 tpi
136 18,000 10-cut woolen singles / 20 tpi
142 20,200 2-ply from 80s
150 22,400 40s Traditional grist for singles used for best sock yarn/ 17 -20 tpi
182 33,600 60s Traditional commercial "fines" / 22- 24 tpi
210 44,800 80s Traditional best commercial "fines" /24 - 27 tpi
A defined ratio between wpi and grist make working with yarn easier. In fact, I would go so far as to say that the textile industry was likely more important to defining various units of measure than the size of the king's foot or finger.
Twist per inch numbers reflect the firmer yarns that I am spinning since starting to spin warp. Before, my 40 count singles were only 17 tpi. These higher tpi were allowed by my move from 25 mm to ~50 mm flier/bobbin whorls.
And, it is all hand spinning. Everything above is plus or minus 10%. I calculate grist by winding hanks of 560 yards and weighing them on a kitchen scale to the nearest gram. In the past, I used a jeweler's scale to weigh fine spinning. Now I just weigh fine hanks 4 at a time and calculate the grist on the basis of grams per 2,240 yards.
ETA - It is worth noting that plying takes up about 10% of the length of the starting singles. Thus, if one starts with 5 hanks of 10s, then one gets just over 500 yards of gansey yarn, and the grist of the 5-ply gansey yarn is is a bit more than the 5 times the grist of the singles. This small change in grist cannot often be measured by wpi.
It works. Mostly it is within 10% of the actual grist, and with corrections for fiber and construction, wpi can measure grist to within 2%, which is better than a yarn balance.
To measure wpi so that it indicates grist, you need a wpi gauge, you need to pack to refusal, and you need to practice. You need to take various kinds of yarns with known grists, and practice doing wpi, until you get the correct grist, time after time. It is a skill.
So, when a knitter says a yarn is 18 wpi, I figure the grist is 324 ypp (e.g., 18x18) -- that is like rope. Then, she gives the yarn band info of 175 yard per 2 oz. -- or 1,400 yards per pound. That is sock or fingering yarn. When I measure the wpi of that yarn, I get 37 wpi. That tells me, and other spinners, the grist.
Packing the yarn to refusal gives a definite measure that can be made anywhere by anyone. Packing the yarn loosely so that a 1,400 ypp yarn so that it yields a wpi of 18 is not a repeatable measure of -- anything. This approach is as much a measure of twist as of grist. With this approach, high twist yarns will seem to have much lower grist than yarns with the same grist, but less twist - and vice versa.
If you pack to refusal, then worsted and woolen yarns of the same grist will have the same wpi. If you use the loose knitter's wpi, then worsted and woolen yarns of the same grist will have very different wpi. If you pack to refusal then yarns of the same grist but different twist will have the same wpi. If you use the loose knitter's wpi, then yarns that are the same grist, but which have different twist will have different wpi. This is silly. What really counts to a knitter is the length and weight of the yarn. These should be described by the wpi. Twist of the yarn is less important.
In my world wpi and grist are related thusly:
wpi grist (ypp) spin count notes
22 484 Aran Yarn (traditionally was 10-ply of 10 count singles)
24 1
26 676
28 840 Worsted Yarn (traditionally was 6-ply of 10 count singles)
30 900
32 1,000 Gansey Yarn ( 5-ply of 10 count singles)
33 1,100 DK weight yarns
34 1,120 2
38 1,443 Common grist for commercial sock yarns e.g., Wooly West
40 1,650 3 My 6-strand sock yarn
42 1,800 Single cut woolen singles
44 1935 Fingering Yarn
48 2,303 Jumper Weight/ Spindrift weight 2-ply
53 2,800 5
60 3,600 2-cut woolen singles @ 9 tpi
60 3,733 7 Traditional 6-ply sock yarn from 40 count singles
64 4,100 Modern lace weight
75 5,600 10s Singles for warp/ 9-10 tpi & woolen singles for weft @ 12 tpi
80 6,700 13 Traditional 3-ply Shetland lace plied up from 40 count singles
105 11,200 20s Worsted singles that I use for my sock yarn @ 14 tpi
120 14,400 8- cut woolen singles / 18 tpi
130 16,800 30s Worsted singles @ 17 tpi
136 18,000 10-cut woolen singles / 20 tpi
142 20,200 2-ply from 80s
150 22,400 40s Traditional grist for singles used for best sock yarn/ 17 -20 tpi
182 33,600 60s Traditional commercial "fines" / 22- 24 tpi
210 44,800 80s Traditional best commercial "fines" /24 - 27 tpi
A defined ratio between wpi and grist make working with yarn easier. In fact, I would go so far as to say that the textile industry was likely more important to defining various units of measure than the size of the king's foot or finger.
Twist per inch numbers reflect the firmer yarns that I am spinning since starting to spin warp. Before, my 40 count singles were only 17 tpi. These higher tpi were allowed by my move from 25 mm to ~50 mm flier/bobbin whorls.
And, it is all hand spinning. Everything above is plus or minus 10%. I calculate grist by winding hanks of 560 yards and weighing them on a kitchen scale to the nearest gram. In the past, I used a jeweler's scale to weigh fine spinning. Now I just weigh fine hanks 4 at a time and calculate the grist on the basis of grams per 2,240 yards.
ETA - It is worth noting that plying takes up about 10% of the length of the starting singles. Thus, if one starts with 5 hanks of 10s, then one gets just over 500 yards of gansey yarn, and the grist of the 5-ply gansey yarn is is a bit more than the 5 times the grist of the singles. This small change in grist cannot often be measured by wpi.
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