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Cutting Depth and Rebar Size Reality
On paper both grinders cut rebar, but usable depth of cut is what matters on site. A 4-1/2 inch grinder with a fresh cutoff wheel gives you about 1 to 1-1/8 inches of usable depth when you account for the arbor, guard, and the fact you never want to bury the nut into the cut. A 7 inch grinder gives you about 2-3/4 inches.
For common rebar that difference is decisive only at a certain size:
#3 (3/8″) and #4 (1/2″) rebar: Either grinder cuts in one pass. A 4-1/2 inch wheel goes through in 2-4 seconds if you let the wheel do the work.
#5 (5/8″) and #6 (3/4″) rebar: Still one-pass for a 4-1/2 inch grinder, but you are using most of the wheel diameter. If the bar is against a form or in a tight cage, the guard will hit before you finish the cut.
#7 (7/8″) and #8 (1″) rebar: A 4-1/2 inch grinder often needs two passes or a roll of the bar. A 7 inch grinder still does it in one pass with clearance to spare. If you cut bundles or dowels sticking out of a footing, that single-pass clearance saves a lot of repositioning.
If your work is mostly cutting #4 and #5 to length on sawhorses, depth is not your limiting factor. If you cut in place, flush against concrete, or cut gang-cut bundles, the 7 inch advantage is real.
Power, Speed, and Real Cut Time
The typical 4-1/2 inch grinder is 7 to 10 amps, 11,000 RPM, and weighs 3.8 to 6 pounds. The typical 7 inch grinder is 13 to 15 amps, 8,500 RPM, and weighs 8.5 to 12 pounds. Larger motor means more torque under load.
In practice, a 4-1/2 inch grinder cuts faster on small rebar because rim speed is higher and the thin 0.045-inch wheel removes less material. On a single #4 bar, the 4-1/2 inch will be slightly quicker. On #6 and up, or when you stack three bars, the 7 inch motor does not bog down and finishes faster because you can lean into it without stalling.
Where the 4-1/2 inch fails on site is heat and duty cycle. Those small motors get hot after 10-15 continuous cuts in #5 rebar on a hot day, especially with a long 100-foot 16-gauge extension cord. Voltage drop will stall a 7-amp grinder long before it stalls a 15-amp grinder on a 12-gauge cord. If you are running off a generator or a far outlet, the bigger grinder is more tolerant, though it will trip a 15-amp breaker if anything else is on the circuit.
Failure mode to watch: pushing a thin cutoff wheel sideways to speed the cut. On a 4-1/2 inch grinder the wheel flexes and shatters. On a 7 inch grinder the wheel is stiffer but kickback is much more violent because of the mass and leverage. Never use the side of a cutoff wheel to deburr or grind; switch to a grinding wheel if you need to dress the cut.
Handling, Weight, and Fatigue
This is where most crews make the wrong choice. A 7 inch grinder cuts bigger material, but you will not want to hold it overhead or one-handed while the other hand holds the bar.
A 4-1/2 inch angle grinder can be run with one hand for a second to stabilize work, it fits between form studs and rebar mats, and you can carry it in a tool belt. For ladder work, wall ties, or cutting stirrups in a column cage, the smaller size is safer because you maintain control.
A 7 inch angle grinder demands two hands, a solid stance, and room for the guard. It is 4 to 6 pounds heavier and produces 2-3x more gyroscopic force, so a twist if the wheel binds will wrench your wrist harder. On the ground cutting 20-foot sticks to length, that weight is fine and even helps keep the cut straight. On a scaffold cutting protruding dowels, it is exhausting after a dozen cuts.
If you have only one grinder for general site work – cutting rebar, grinding concrete lips, wire wheeling – the 4-1/2 inch gets used 90% of the time because it is there and ready. The 7 inch stays in the gang box until you need it.
Disc Cost, Life, and Consumables
Both sizes use 0.045-inch thin cutoff wheels for rebar, but cost and life differ. A good 4-1/2 inch cutoff wheel is $1 to $2.50 in packs and lasts 15-25 cuts in #4 rebar before it is down to 3-1/2 inches and losing depth. A 7 inch wheel is $3.50 to $6 and lasts 30-50 cuts because there is more abrasive to wear.
Per cut, cost is similar, but the 7 inch wheel is less likely to run out of diameter mid-job. That matters if you are cutting #6 all day; a worn 4-1/2 inch wheel that started at 4.5 inches will not make a one-pass cut anymore at 4 inches.
Sparks and noise are also different. A 7 inch wheel throws a much larger and hotter spark stream and is noticeably louder. Inside a foundation or parking garage you will want a full face shield, not just glasses, and you need to plan spark direction away from form oil, foam, and other trades. Both sizes need hearing protection, but the 7 inch at full load is in the 100-105 dB range.
For cleanest cuts in rebar, look for metal cutoff wheels for rebar rated specifically for steel and for your grinder’s RPM. Avoid cheap masonry/combo wheels; they cut slower, create more heat, and temper the rebar end blue, which can affect bending nearby.
Head-to-Head Comparison
| Spec | 4-1/2 Inch Grinder | 7 Inch Grinder |
|---|---|---|
| Usable Depth of Cut | 1″ – 1-1/8″ | 2-1/2″ – 2-3/4″ |
| Motor | 7-10 Amp, ~11,000 RPM | 13-15 Amp, ~8,500 RPM |
| Weight (tool only) | 3.8 – 6 lbs | 8.5 – 12 lbs |
| Wheel Cost | $1.00 – $2.50 | $3.50 – $6.00 |
| Cuts Per Wheel (#4 rebar) | 15 – 25 | 30 – 50 |
| Best For | One or two bars, in-place cages, overhead, tight access | Bundles, #7+ bar, flush cuts to concrete, production sawing on ground |
| Weak Point | Bogs and overheats on thick/bundled bar, loses depth as wheel wears | Heavy, more dangerous kickback, needs 20A circuit and two hands |
Which Should You Buy for Rebar Work?
Buy the 4-1/2 inch if you cut #3 to #6, work alone, or cut in cages and forms. It is cheaper, lighter, and you will actually use it. Most residential and light commercial concrete work lives here. Get a grinder with a paddle switch and a good guard you can rotate without tools; that matters more than another amp of power. Run thin 0.045-inch wheels and let the weight of the tool feed the cut instead of forcing it.
Buy the 7 inch if you regularly cut #7 and larger, cut multiple bars at once, or need flush cuts against slabs and walls where guard clearance matters. It also makes sense if you already own a 4-1/2 inch for detail work and want a station grinder that lives on sawhorses for cut-to-length production.
For many site crews the right answer is both: a 4-1/2 inch cordless or corded grinder on your person for everything, and a corded 7 inch in the trailer for the days you gang-cut. If you can only have one corded tool and your rebar is #6 and under, spend the savings on quality cutoff wheels, a 12-gauge extension cord, and a face shield. Those three improve cut quality and safety more than stepping up an inch in disc size.
One last note: neither grinder replaces bolt cutters or a rebar shear where you can use them. Cutting with an abrasive wheel leaves a hot, sharp burr. If the bar will be handled or touched by other trades, take two seconds with a flap disc or file to knock the burr off. It prevents glove tears and makes tying faster.