Three units, one product
Bulk trimmer line is sold by weight. The head is loaded by length. Consumption happens per hour of work. Converting between them needs the density of the material and the cross-section of the line, and both are available: nylon monofilament runs about 1.14 grams per cubic centimetre, and a round line of diameter d has a cross-sectional area of pi times d squared over four.
Run that out and the common sizes land close to figures that people who buy a lot of line already know by heart. A pound of 0.065 is roughly 610 feet, 0.080 is about 400, 0.095 about 285, 0.105 about 234, and 0.130 about 152. Those numbers agree with published spool lengths to within a few percent, which is as close as anything gets in this trade. The reason they matter is that the price comparison between a three pound spool of one size and a one pound spool of another is meaningless until both are in feet.
| Diameter | Metric | Feet per lb | Feet in a 3 lb spool | Usual home |
|---|---|---|---|---|
| 0.065 in | 1.65 mm | about 610 | about 1,830 | Light electric and cordless |
| 0.080 in | 2.0 mm | about 400 | about 1,200 | Small gas, larger cordless |
| 0.095 in | 2.4 mm | about 285 | about 855 | The common homeowner size |
| 0.105 in | 2.7 mm | about 234 | about 700 | Heavier gas trimmers |
| 0.130 in | 3.3 mm | about 152 | about 456 | Commercial straight shaft |
| 0.155 in | 4.0 mm | about 107 | about 321 | Brushcutters, clearing work |
The jump from 0.095 to 0.130 more than halves the footage for the same money. That is worth knowing before deciding that heavier line is obviously better, because the heavier line also has to be justified by what it is cutting and by what the machine is rated to swing.
Working out what a head holds
Pull the spool out of the head and take three measurements: the diameter of the solid hub the line winds onto, the diameter across the flanges, and the total depth of the winding channel. On a two-chamber spool that keeps the two lines separated, add the depths of both chambers together, because the calculation only cares about total volume.
The available volume is the annulus between hub and flange multiplied by the depth. Line does not fill that solid; a tidy hand wind achieves roughly 80 to 85 percent, and a hurried one in the field is worse. The last few feet are also the ones that jam, because they sit against the flange where the line wants to cross over itself. If the calculated capacity comes out well above what you can actually get in, drop the packing figure rather than assume the measurement was wrong.
Consumption is about what you hit, not what you cut
Grass does not wear line out at any meaningful rate. What consumes line is abrasion and impact against hard things: concrete edges, gravel, chain link, brick, wire, tree bark and fence posts. A clean turf edge along a bed might use twenty to forty feet in an hour. The same hour spent along a chain link fence line with gravel underneath can burn two hundred, and no line construction changes that by more than a modest fraction.
The two changes that do move it are technique and geometry. Cutting with the last inch or two of the line rather than the middle keeps the working section away from the obstacle and cuts faster, because the tip is where the speed is. Standing back and letting the tip reach in, rather than driving the head at the fence, is the whole difference. The second change is at the property rather than the machine: a mow strip or a hard edge that a mower deck can follow removes the trimmer pass entirely, and that shows up in the mowing time calculator as time recovered on every visit.
Where these numbers stop being right
Everything above assumes round nylon monofilament. Twisted, square, star and serrated profiles have different cross-sectional areas at the same nominal size, so their feet per pound differs, usually less than the round figure because the nominal size describes the widest dimension. Co-polymer and reinforced lines carry different densities again. Whenever the packaging states a length, use the stated length; this calculation is for the spools that only tell you a weight.
Line diameter is also not a free choice. Every trimmer has a maximum, set by the head, the shaft and the power available, and it is in the manual. Oversize line makes the engine work harder for every revolution, loads the gearbox on a straight shaft machine, and on a curved shaft trimmer will simply bog. If line size is being changed because the machine is struggling, the answer is usually the other direction. For what the engine itself needs on a schedule, the service interval planner covers it, and a two-stroke trimmer takes its mix from the two stroke fuel mix calculator.
Questions people ask
How many feet are in a 3 lb spool of 0.095 trimmer line?
About 855 feet, from the geometry of round nylon monofilament at roughly 1.14 grams per cubic centimetre. Published lengths for 3 lb spools of 0.095 usually sit between 820 and 860 feet, and the small spread comes from the exact polymer blend and how tightly the maker holds the diameter tolerance. If your spool has a stated length on the label, that figure is better than this one. Where the calculation earns its keep is comparing a spool sold only by weight against one sold only by length, which is the usual situation on a shelf.
What line diameter should I use?
The size your trimmer is rated for, which is in the manual and often moulded into the head itself. Heavier line is not automatically better: it takes more power to swing, it loads the gearbox and clutch on a straight shaft machine, and it will bog a small engine or drain a battery noticeably faster. What heavier line buys is resistance to being destroyed by hard obstacles and the ability to cut thicker stems without stalling, both of which are real if the work calls for it. If the trimmer bogs in ordinary grass, the diameter is probably already too large for that machine.
Why does my line break constantly at the head?
Usually one of three things. The line has been sitting dry and has become brittle, which is common with line stored through a hot summer or over a couple of years; soaking it in water for a day before winding is a long-standing trick that restores some flexibility. Or the eyelets in the head are worn to a sharp edge and are cutting the line as it feeds, which is a cheap part to replace and is worth checking. Or the working section is being driven into a hard obstacle instead of reaching it with the tip. All three produce the same symptom, so check the eyelets first because it takes a moment.
Is it worth buying bulk line rather than pre-wound spools?
On cost per foot, bulk wins by a wide margin, and the gap grows the more you use. The trade is your time and a tidy wind, because badly wound line welds to itself under heat and stops feeding, which costs more minutes in the field than it saved at the counter. The usual compromise for people who trim a lot is bulk line with several spare spools wound at the bench in advance, so a change in the field is a spool swap rather than a winding job. Put your own numbers into the calculator with the cost field filled and the season figure will tell you whether the difference is worth the handling.
Does trimmer line go bad in storage?
It dries out. Nylon absorbs moisture from the air and holds a small amount of it, and that water content is part of what makes the line flexible rather than brittle. Line stored in a hot dry shed for a season or two comes out noticeably more prone to snapping off short at the head. Storing it in a sealed container, ideally somewhere that does not bake, slows it down, and soaking an old spool in water for a day or two before winding brings back a useful amount of the flexibility. It is not a permanent fix and very old line is better replaced.