DC Motor Speed Calculator

This tool calculates the loaded motor speed (RPM), output torque, mechanical power, electrical input power, and overall efficiency—both before and after gear reduction. It also estimates current draw and back-EMF voltage based on linear DC motor characteristics.

The gear ratio multiplies output torque and reduces output speed proportionally (e.g., 10:1 increases torque by 10× and decreases speed by 10×). Efficiency losses from gearing are not modeled by default but can be approximated by manually reducing final efficiency by 1–5% per stage.

No-load speed (n₀) is the motor’s maximum RPM at rated voltage with zero load torque. Stall torque (Tₛ) is the maximum torque the motor produces when locked (0 RPM) at rated voltage. These define the motor’s ideal linear speed-torque curve: n = n₀ × (1 − T/Tₛ).

Small DC motors (e.g., brushed coreless or planetary-gear types) commonly specify stall and load torques in millinewton-meters (mN·m) for precision—typical values range from 10 to 500 mN·m. Entering in N·m would require decimals (e.g., 0.03 N·m), increasing input error risk.

Common 12 V hobby/robotics motors have no-load speeds of 3,000–10,000 RPM and stall torques of 20–200 mN·m. Industrial DC motors may operate at lower speeds (1,000–3,000 RPM) with higher torque (500–5,000 mN·m). Always verify specs from manufacturer datasheets.

Yes—the tool treats effective voltage as proportional to duty cycle. For example, a 12 V supply with 60% PWM yields ~7.2 V equivalent; enter that as “Rated Voltage” to estimate speed/torque under PWM. Note: This assumes ideal average-voltage behavior and ignores inductive ripple effects.

Efficiency peaks near 30–50% of stall torque and falls to near zero at stall because mechanical output power (T × ω) drops to zero while resistive losses (I²R) remain high. This reflects real motor physics—always avoid continuous operation above 80% of stall torque without active cooling.

Results assume ideal linear motor characteristics and neglect temperature rise, brush contact resistance, and magnetic saturation. For design validation, derate torque by 10–15% and speed by 5–10% to account for real-world losses and voltage sag under load.

It models a single overall gear ratio. For multi-stage gearboxes, multiply individual ratios (e.g., 5:1 × 4:1 = 20:1) and enter the total. Efficiency impact is cumulative—assume ~95% per stage, so two stages reduce total efficiency by ~10% versus single-stage.

Motor speed refers to the armature’s rotational speed before gearing; output speed is the shaft speed after applying the gear ratio. Both are shown separately to help validate gearbox selection and ensure mechanical limits (e.g., max RPM of gears or encoders) aren’t exceeded.