Router RPM & chip load helper
"Just use the router's max speed" is bad advice — a 1/4″ bit in soft pine at 25,000 RPM is burning the wood; a 3″ panel-raising bit at 25,000 RPM in hard maple is too slow and chattering. The right speed depends on bit diameter, material and the number of cutting edges. This calculator does that maths.
Calculator
Surface feet per minute (SFM), briefly
SFM is how fast the cutting edge sweeps past the workpiece at the rim of the bit. Wood cuts cleanly in a narrow band — too slow and you get rubbing and burning; too fast and you get tear-out and chatter. The numbers in the material dropdown are conservative targets; you can usually push +20% without trouble.
Chip load (per flute per revolution) for a hand-guided router typically lands in the 0.005–0.015″ range for hardwoods, 0.010–0.020″ for softwoods. The calculator's "feed" guidance assumes you can keep that rate while watching the cut, not forcing the bit.
Why big bits need slow routers
A 3″ panel-raising bit at 25,000 RPM has a rim speed of 19,600 FPM — way past where wood cuts cleanly. Large bits need slower spindles (a CNC router spindle runs 6,000–18,000 RPM for this reason). Most plunge routers can't go that slow; a dedicated mid-speed router or a router table with a variable-speed controller is the right answer for large bits.
Limitations
- SFM is a starting point. Bit geometry (number of wings, helix angle, carbide grade) matters as much as diameter.
- Doesn't model chip load for CNC-rate feed. For CNC, use a feed-and-speed calculator tuned to your controller.
- Assumes sharp bits. A dull bit needs slower speed and slower feed to avoid burning.
- Doesn't capture dust extraction — connected extraction lets you run faster than freehand.
Frequently asked questions
Burning at the cut line — speed or feed?
Usually feed. The bit is spending too long in contact with the wood — push faster, slow down only if pushing faster doesn't help. Burning with a sharp bit at the recommended speed almost always means the cut is too deep or the feed is too slow.
Can I just use the router's max RPM for everything?
For 1/4″ bits in softwood, yes — close enough. For larger bits or denser woods, no. Smaller-diameter bits need higher RPM to reach cutting speed; larger bits need lower RPM to avoid rim-speed burning.
Why does my flush-trim bit always chatter?
Common causes: bearing is worn, bit is bent (dropped the router), feed rate is wrong for the bit, or the workpiece isn't well-supported. The calculator gives you a target speed — chattering usually goes away by combining correct speed with slower feed and a fresh bit.