How Many Cuts Will a 5Ah Battery Give a Circular Saw?

So the 537 and 610 you see on Milwaukee and Makita pages are not three saws beating DeWalt by half. They are bigger batteries: Milwaukee's figure sits on a kit with 9.0 Ah packs, and Makita's saw runs on two 5.0 Ah packs at once. Put every claim on the same 90 Wh pack and the spread shrinks to about 60 cuts.

What a cuts per charge figure means

A cuts per charge figure is the number of times a saw cuts fully through a board before one full charge runs out. On every page checked here the cut is a crosscut, a cut across the board from one edge to the other, so a crosscut in a 2x4 goes through the 3-1/2 in face. The figure does not say how long the board is, only how many times the blade passes through it.

The figure belongs to the saw and the battery together, so a bigger pack on the same saw raises the count. That is why a count on its own compares nothing until it is divided by the size of the battery it was made on.

Why watt hours, not amp hours

A watt hour (Wh) is a unit of energy: how much work a battery can do, the way gallons measure what a fuel tank holds. Federal rules define a lithium-ion pack's rating as its amp hours times its nominal voltage, in 49 CFR 171.8.

Amp hours (Ah) alone are not enough here, because Makita's 36 V saws run on two 18 V packs, so 5.0 Ah on that saw holds twice the energy of 5.0 Ah on an 18 V saw. Watt hours fold both numbers into one.

Use the nominal voltage, not the number on the front of the pack. DeWalt's own footnote says a 20V MAX pack is 18 V nominal, and Milwaukee M18, Makita 18V LXT and Ryobi 18V ONE+ packs are listed at 18V, so every pack on this page is 18 V. The full story of the 20 V label is in is 20V MAX the same as 18V.

What each maker claims, on which pack

These are the claims as printed, with the lumber and the battery each page gives. SPF stands for spruce-pine-fir, the common species group for framing lumber.

Two of these need a note. DeWalt and Milwaukee print their claims on kit pages, and neither claim names the pack in the same sentence. The 330 sits on a page whose specification lists one battery at 5.0Ah. The 537 sits on a page whose kit holds two HIGH DEMAND 9.0 packs, and it says "on one charge," so one 9.0 pack. This page reads each claim against the pack in its own kit, which is a reading and not a maker's statement.

Ryobi's newer PBLCS302 page gives 370 cuts and no battery or lumber at all. A count with no pack behind it cannot be put on a per Wh scale, so it is left out of the next table.

No maker publishes the test method

None of the pages checked here says how its count was made. Makita says "internal testing," and Skil says "lab testing" for its 600 cuts in 2x4 SPF on two 5.0Ah packs. Skil is left out of the tables because the page checked here gives no nominal voltage for its packs. DeWalt, Milwaukee and Ryobi give the lumber and nothing else.

No page states the blade, its tooth count or whether it was new. None gives the depth setting, how fast the saw was pushed through the wood, or whether there was a rest between cuts. None gives the moisture in the lumber or the temperature of the pack, or says whether "per charge" ended when the saw stopped or when it slowed. Every one of those changes the count, and none can be checked.

That does not make the figures useless. Each is a maker's own result, measured its own way, and Milwaukee and Makita print theirs as "up to," a ceiling. Dividing by watt hours removes the one difference that is published, the battery, but it cannot remove the rest.

Every claim in cuts per watt hour

Read down the cuts per Wh column. The largest claim on the page, Makita's 610, works out to less per watt hour than DeWalt's 330, and so does Milwaukee's 537. The highest rate here is 3.7 and the lowest is 3.0, about 22 percent apart, which is narrow against a raw spread of 325 to 610.

The spread that is left cannot be pinned on the saws, because the lumber differs, from pine to SPF to unnamed, and the methods are unpublished. A 0.7 cut per Wh gap between two makers is smaller than anything a reader could check. Treat 3.0 to 3.7 as one band.

Bigger boards cost more per cut

Makita is the only maker here that gives a second lumber size, and both of its saws list a 2x10 SPF count beside the 2x4 count, on the same two packs.

Both saws make 0.52 times as many 2x10 cuts as 2x4 cuts. So a 2x10 crosscut costs about twice the energy of a 2x4 crosscut, which is less than the board size suggests: a 2x10 face is 9-1/4 in wide against 3-1/2 in, 2.6 times the cut. Makita does not say why the cost rises more slowly than the width. Only one maker publishes this pair, so treat the 0.52 as Makita's figure, not a rule for every saw.

A worked case: one 5.0 Ah pack and a deck frame

Take an 18 V 5.0 Ah pack on any of these saws. Suppose the job is 60 crosscuts in 2x10 joists and 100 in 2x4 blocking. Will one charge do it?

Start with the pack, which holds 18 V x 5.0 Ah = 90 Wh.

Price the 2x4 cuts at the low end of the band, 3.0 cuts per Wh, so 100 / 3.0 = 33.3 Wh.

Price the 2x10 cuts at Makita's lower 2x10 rate, 1.6 cuts per Wh, so 60 / 1.6 = 37.5 Wh.

Added together, 33.3 + 37.5 is about 71 Wh out of 90. On paper the pack covers the job with about 19 Wh left, roughly 55 more 2x4 cuts.

Worked through at the high end instead, 3.7 and 1.8 cuts per Wh, the same job needs 27 + 33 = 60 Wh, so either way one pack covers it on paper. If the margin had come out near zero, the answer would be a second pack, not a closer reading of the claims. A spare that has to fly with you has a separate limit, set out in flying with power tool batteries.

How to read a claim that is not on this page

New saws come out with new counts, and the box rarely does the division for you. Four questions turn any claim into a number you can use.

1. Which pack? Look in the claim, then in its footnote, then in the kit contents. If none of them names a pack, as with Ryobi's 370, the count cannot be compared.
2. How many packs? A saw that takes two packs uses both, so count the energy in each.
3. What nominal voltage? Use the footnote or the pack's own page, not the number on the front.
4. What lumber? A 2x4 count and a 2x10 count are not the same job, and Makita's figures put one 2x10 cut at about two 2x4 cuts.

Then divide the count by the energy. If the answer lands well above 3.7, check the pack count and the lumber again, since a missed second pack or a thinner board makes a count look better than it is. A claim far below 3.0 is either a harder test or a saw that uses more energy per cut, and the page will not say which.

Your own pack, in cuts

Multiply your pack's nominal volts by its amp hours, then multiply that by 3.0 and by 3.7. Those two numbers are the makers' band for 2x4 crosscuts, and the lower one is the one to plan on.

The 5.0 Ah row tops out at 333 because 3.7 is rounded, against DeWalt's own 330 on that pack. The 9.0 Ah row brackets Milwaukee's 537.

A saw on a 36 V X2 body takes two packs, so add their energy before you multiply. Two 18 V 5.0 Ah packs make 180 Wh, which is why Makita's counts sit near double the 5.0 Ah row.

If you also have a 12 Ah pack, the cut count is only half the planning, and the other half is how long it takes to put the charge back, worked out in how long a 12Ah battery takes to charge.

Where this answer stops applying

These rates come from maker claims made by unpublished methods, and they describe 7-1/4 in saws crosscutting softwood framing lumber only. Rip cuts along the board, plywood, hardwood, wet lumber, a dull blade and bevel cuts are all outside them. No page here gives a figure for any of those, so this page does not either. The Wh rating is nominal volts times rated capacity, not the energy a pack actually delivers under a saw's heavy draw, and nothing checked here says whether a small pack and a large pack give up the same share of it. A cold pack gives up less, as the battery storage temperature guide explains, so a winter morning count will fall short of every row above. Standard and rear handle saws, 18 V and 36 V, all sit in the band, but only five saws were checked and a saw not listed may fall outside it. A corded saw never runs out of cuts. If the count matters more than the cord, a generator answers it, sized in what size generator runs a 15 amp table saw, which also lists a circular saw's running and starting watts.

The thirty-second check

1. Find your pack's nominal voltage, which on DeWalt 20V MAX, Milwaukee M18, Makita LXT and Ryobi ONE+ is 18 V.
2. Multiply by the amp hours. A 5.0 Ah pack is 90 Wh. On a two-pack saw, add both packs.
3. Multiply the Wh by 3 for a planning count of 2x4 crosscuts at the low end of the makers' claims.
4. For 2x10, halve it, going by Makita's two saws.
5. If the job comes close to that count, bring a second pack.