An 18 inch chain does not exist
Bar length tells you how much wood the saw can reach across. It tells a parts counter almost nothing, because the loop that fits an 18 inch bar depends on how far apart the rivets sit. Pitch is that spacing, and at .325 pitch an 18 inch bar takes noticeably more drive links than the same bar length at 3/8, simply because each link covers less distance. The two chains are not interchangeable in either direction: the wrong pitch will not mesh with the drive sprocket, and forcing the issue damages the sprocket rather than the chain.
Gauge is the third number and it is the thickness of the drive link tang that rides in the bar groove. A .050 chain in a .063 groove flops sideways and cuts a curve. A .063 chain will not go into a .050 groove at all. Gauge and pitch are almost always stamped into the bar near the mounting slot, along with the drive link count, and reading those three stamps takes ten seconds and removes all the guesswork this page is trying to approximate.
Why the link count here is an estimate
The loop has to cover the bar out and back plus the wrap around the drive sprocket and the nose sprocket, plus the length of bar hidden under the clutch cover. That hidden allowance is a design choice made by whoever built the bar, and it varies between makers and between families from the same maker. Two bars both sold as 20 inch can call for chains three or four drive links apart.
The calculation here divides bar length plus an allowance by the pitch, and the allowance is the soft part. That is what the calibration fields are for: enter one bar you own with its stamped drive link count and the tool solves for the allowance your bars actually use, then applies it to the bar you are asking about. Without a calibration, treat the number as a range to check a purchase against rather than a specification to order from.
| Pitch | Usually found on | Round file | Typical depth gauge |
|---|---|---|---|
| 1/4 in | Carving bars, top handle saws | 5/32 in (4.0 mm) | .020 to .025 in |
| 3/8 in low profile | Small homeowner saws, marked LP or Picco | 5/32 in (4.0 mm) | .025 in |
| .325 in | Mid-size homeowner and farm saws | 3/16 in (4.8 mm) | .025 in |
| 3/8 in standard | Larger saws, most professional work | 7/32 in (5.5 mm) | .025 to .030 in |
| .404 in | Large professional saws, milling | 7/32 in (5.5 mm) | .030 in |
Note that 3/8 low profile and 3/8 standard share a nominal pitch but not a chain. The low profile cutter is shorter, the drive links are different, and the file size is not the same. If a chain is marked LP, Picco or with a low profile part number, it belongs with a matching bar and sprocket.
The depth gauge is what most people never touch
In front of each cutter sits a small ramp, the depth gauge or raker, and its height below the cutter tip decides how thick a chip that cutter can take. Every time the cutter is filed back it gets shorter, the tooth profile drops slightly, and the effective bite reduces. A chain that has been sharpened a dozen times with a file and never had the depth gauges lowered will be perfectly sharp and will still ride on the rakers and produce dust instead of chips.
The correction is a flat file and a depth gauge tool, which is a slotted plate that exposes only the material that should come off. It costs very little and it is the difference between a chain that pulls itself into the cut and one you have to lean on. The setting is a specification from the chain maker for that chain, and it changes with pitch and with the type of cutter, so the figures in the table above are orientation and not authority.
Sharp is a safety subject, not a convenience one
A dull chain or one filed unevenly is more dangerous than a sharp one. It requires force, force removes control, it bounces rather than bites, and uneven cutters pull the bar to one side in the cut. Kickback, chain brake function and cutting technique are training subjects that belong with hands-on instruction and protective equipment, not with a web page, and nothing here is a substitute for either. What this calculator covers is the parts identification and the filing consumables, which is where the money and the confusion go.
If you are cutting firewood with the results, the firewood cord calculator handles the volume side of the pile. The saw itself runs on mixed fuel, so the ratio comes from the two stroke fuel mix calculator, and the maintenance that gets forgotten between cutting seasons is laid out in the small engine service interval planner.
Questions people ask
How do I find my drive link count without a calculator?
Three places, in order of reliability. First, the stamp on the bar itself, near the mounting slot, which usually carries the length, the pitch, the gauge and the drive link count together. Second, the box or the tie-band label of the chain currently on the saw, if you still have it. Third, count them: lay the loop flat, mark a starting link with a piece of tape and count every tang that points down into the groove, all the way around. It takes a couple of minutes and it is the only method that is certain, which is why shops do it when a customer arrives with a bare loop and no information.
What is the difference between pitch and gauge?
Pitch is the spacing along the chain, measured as the distance between any three consecutive rivets divided by two, and it has to match the drive sprocket and the nose sprocket. Gauge is the thickness of the drive link tang that rides inside the bar groove, and it has to match the bar. They are independent: you can have .325 pitch at .050 gauge or at .063 gauge, and both are common. When someone buys the wrong chain it is usually gauge, because pitch failures are obvious the moment you try to fit it and a gauge mismatch will sometimes go on and then cut badly.
Which file size do I need for my chain?
It follows the pitch and the cutter type, not the bar length. As a general orientation, 1/4 pitch and 3/8 low profile take a 5/32 inch (4.0 mm) round file, .325 takes 3/16 inch (4.8 mm), and 3/8 standard and .404 take 7/32 inch (5.5 mm). Some chain families specify something different, particularly low-kickback and semi-chisel variants, so the chain maker figure for your part number is the one that governs. A file that is too small drops the top plate angle and produces a hook that grabs; too large and the cutter never gets a proper edge.
Can I put a longer bar on my saw?
Sometimes, and the question has more parts than chain sizing. The mount has to match, the chain has to be the right pitch for the drive sprocket that is already on the saw, the oiler has to deliver enough to reach the nose of the longer bar, and the engine has to have enough power to pull the extra cutters through wood. Fitting a bar that the saw can physically accept but cannot lubricate or drive is how people burn a nose sprocket or run a chain hot. Manufacturers publish a maximum bar length per model and it is a real limit, not a marketing one.
How often should the depth gauges be lowered?
People commonly do it every few sharpenings rather than every one, and check with a depth gauge tool each time rather than counting. The reason it varies is that how far a cutter drops depends on how much steel you are taking off, which depends on how dull the chain was allowed to get and whether it hit dirt. The symptom that it is overdue is a chain that is visibly sharp and still produces fine dust rather than chips and needs pressure to feed. The symptom of going too far is a chain that grabs and chatters. A slotted depth gauge tool removes the judgement from it entirely and costs less than one chain.