Drill Speed Calculator: RPM & Feed Rate

A drill speed calculator works out the correct spindle speed (RPM) and sometimes feed rate for a given drill bit diameter and material. You enter the bit diameter, the material you are drilling, and the type of operation (e.g., drilling vs. reaming), and it returns the recommended revolutions per minute and, if you provide feed per revolution, the feed rate in mm/min or inches/min.

Formula

The fundamental formula for spindle speed is:

N = (V × 1000) / (π × D)

• N = spindle speed in revolutions per minute (RPM)
• V = cutting speed in meters per minute (m/min) – a material-specific constant
• D = drill diameter in millimeters (mm)
• Ï€ ≈ 3.14159

If you work in inches, the formula becomes N = (V × 12) / (π × D_in), where V is in surface feet per minute (SFM) and D_in is in inches. The calculator above uses metric units, but you can convert by multiplying inches by 25.4.

Worked Example

Using the default values from the calculator: a 10 mm drill bit, cutting speed 30 m/min (typical for mild steel).

1. N = (30 × 1000) / (π × 10) = 30000 / 31.4159 ≈ 954.9 RPM
2. The calculator rounds to 955 RPM.
3. Feed rate (assuming 0.1 mm/rev) = 955 × 0.1 = 95.5 mm/min.

So for a 10 mm drill in mild steel, set the spindle to about 955 RPM and feed at roughly 95 mm/min.

What Is Cutting Speed?

Cutting speed (V) is the speed at which the outer edge of the drill moves through the material. It is not the RPM itself; it is the linear speed of the cutting edge. Different materials have different optimal cutting speeds. For example:

• Aluminum: 60–120 m/min (200–400 SFM)
• Mild steel: 30–40 m/min (100–130 SFM)
• Stainless steel: 10–20 m/min (30–65 SFM)
• Brass: 60–90 m/min (200–300 SFM)
• Cast iron: 20–30 m/min (65–100 SFM)

These values are starting points; actual speeds depend on the exact alloy, tool coating, and machine rigidity.

How to Choose the Right Cutting Speed

Selecting the cutting speed is the first step. You can find tables in machining handbooks or on tool manufacturer websites. If you are unsure, start with a lower speed and increase gradually. For a given material, the cutting speed is the same regardless of drill diameter – that is why the formula divides by diameter to get RPM. Smaller drills need higher RPM, larger drills need lower RPM.

Feed Rate and Why It Matters

Feed rate is how fast the drill advances into the material. It is usually given in mm per revolution (mm/rev) or inches per revolution (IPR). The feed rate affects chip thickness and heat generation. A common rule of thumb for HSS twist drills is feed = 0.001–0.002 inches per revolution for small diameters, up to 0.010–0.020 for large diameters. Too high a feed can break the drill; too low a feed causes rubbing and work hardening (especially in stainless steel).

Common Mistakes and Misconceptions

• Using RPM instead of cutting speed: A common error is to use a fixed RPM for all drill sizes. That is wrong – RPM must change with diameter.
• Mixing units: The formula requires consistent units. If you enter diameter in inches but cutting speed in m/min, you get nonsense.
• Ignoring material condition: Harder materials (like hardened steel) require lower cutting speeds. Coatings (TiN, carbide) can allow higher speeds.
• Forgetting the peck: Deep holes need peck drilling to clear chips; the calculator does not account for that.

Limitations of the Simple Formula

The basic formula assumes a standard HSS twist drill and a rigid setup. In practice, you may need to adjust for:

• Drill material: carbide can run 2–3× faster than HSS.
• Hole depth: deeper holes require lower speeds and pecking.
• Machine condition: older machines may vibrate at high RPM.
• Coolant: flood coolant allows higher speeds than dry drilling.

Always start with the calculated speed and reduce if you hear squealing or see excessive wear.

Frequently Asked Questions

What is the difference between cutting speed and spindle speed?

Cutting speed is the linear velocity of the drill edge (m/min or SFM). Spindle speed is how fast the drill rotates (RPM). They are related by the diameter: spindle speed = cutting speed divided by circumference.

How do I convert SFM to RPM?

Use the formula RPM = (SFM × 12) / (π × D_in). For example, 100 SFM with a 0.25-inch drill gives RPM = (100 × 12) / (π × 0.25) ≈ 1528 RPM.

What is a good feed rate for drilling steel?

For a 10 mm HSS drill in mild steel, a feed of 0.1 mm/rev is typical, giving about 95 mm/min at 955 RPM. For smaller drills, reduce feed proportionally.

Can I use the same speed for a center drill or a reamer?

No. Center drills and reamers should run at about half the speed of a twist drill. Reamers need lower feed to avoid chatter.

Why does my drill bit smoke even when I use the calculator?

Smoking usually means the feed is too low (rubbing) or the speed is too high for the specific material. Check the cutting speed table for your exact alloy, and make sure you are using a sharp bit.

Does the calculator work for masonry or wood bits?

No, this formula is for metal-cutting twist drills. Masonry bits use impact and different parameters; wood drills can run at much higher speeds (often 2–3× the metal speed).

Related

• Speed Calculation Questions: How to Solve Them
• Speed Calculation Test: How to Measure It
• Vessel Speed: How to Calculate It (knots)

in d | Drill diameter | mm | 10 | 0
in v | Cutting speed | m/min | 30 | 0
out rpm | Spindle speed | RPM | (v * 1000) / (pi * d) | 0
out feed | Feed rate (0.1 mm/rev) | mm/min | rpm * 0.1 | 1