Robotshop Motor Sizing Calculator

Calculator

Unit System

Chassis Mass (kg)

Payload Mass (kg)

Target Speed (m/s)

Acceleration (m/s^2)

Wheel Diameter (mm)

Slope Grade (%)

Rolling Coefficient

Drag Area CdA (m²)

Air Density (kg/m³)

Driven Motors

Gear Ratio

Gear Efficiency (%)

Motor Efficiency (%)

Safety Factor

Supply Voltage (V)

Candidate Motor Torque (N·m)

Candidate Motor Speed (rpm)

Current Limit Per Motor (A)

Reset

Formula Used

The calculator estimates total driving force first.

F = m × a + m × g × Crr × cos(θ) + m × g × sin(θ) + 0.5 × ρ × CdA × v²

Wheel torque is then calculated from force and wheel radius.

Twheel = F × r

Motor shaft torque includes gear ratio, gear efficiency, and safety factor.

Tmotor = (Twheel per motor ÷ (gear ratio × gear efficiency)) × safety factor

Motor speed is based on wheel circumference and gear ratio.

Motor rpm = (v ÷ (π × D)) × 60 × gear ratio

Estimated current comes from electrical power per motor.

I = P electrical ÷ V

How to Use This Calculator

Select the unit system before entering values.
Enter chassis mass and payload mass.
Add target speed, acceleration, wheel diameter, and slope.
Enter rolling resistance and drag values.
Add motor count, gear ratio, and efficiency values.
Enter candidate motor ratings for pass checks.
Press Calculate to show results above the form.
Download the CSV or PDF report when needed.

Example Data Table

Robot Type	Mass	Speed	Wheel Diameter	Grade	Driven Motors	Gear Ratio
Indoor Rover	15 kg	1.2 m/s	150 mm	10%	2	20:1
Warehouse Cart	45 kg	0.8 m/s	200 mm	6%	4	30:1
Outdoor Platform	28 kg	1.5 m/s	250 mm	15%	4	25:1

Sizing Guide

A robot motor must match the robot, not the catalog picture. This calculator gives a practical first pass for wheeled designs. It combines mass, payload, speed, acceleration, wheel size, grade, rolling drag, and gearing. The result is a motor torque and speed target that can be compared with a real data sheet.

Input Quality

Good sizing starts with honest inputs. Use the full operating mass. Include batteries, mounts, tools, and any payload. Use the worst slope the robot must climb. Use a realistic acceleration value. Small indoor robots may use a low rolling coefficient. Outdoor wheels, carpet, and soft surfaces need a higher value.

Formula And Margins

The main force equation adds acceleration force, rolling resistance, grade force, and air drag. Wheel torque is found by multiplying force by wheel radius. Gear reduction then changes the torque seen by the motor shaft. A higher gear ratio raises wheel torque, but it also raises the motor rpm needed for the same ground speed. Efficiency losses are included because gears, belts, bearings, and tires waste power.

The safety factor is important. Motors heat up during long runs. Starting from rest can demand more torque than steady travel. A factor near 1.5 suits clean indoor movement. A factor near 2 or higher is better for ramps, bumps, and unknown surfaces. The candidate motor fields help check torque, rpm, and current margins.

Using Results Safely

Use the calculator as a design filter. It is not a replacement for testing. Data sheets can list stall torque, no-load speed, rated torque, or peak torque. Continuous torque is usually the safest comparison for long driving. Stall torque should not be used as a normal operating target. Running near stall can overheat a motor quickly.

After you calculate, export the report. Save the CSV for spreadsheets. Save the PDF for build notes. Compare several wheel sizes and gear ratios. Smaller wheels reduce torque demand. Larger wheels can improve obstacle handling. More driven motors split the load. Better gear efficiency lowers current demand. The best design balances torque, rpm, heat, traction, battery life, and available space. Recheck values after choosing tires and batteries. Real weight often changes late. A small update can change torque, current. Later.

FAQs

What does this motor sizing calculator estimate?

It estimates force, wheel torque, motor shaft torque, motor rpm, mechanical power, electrical power, and current per motor. It also checks candidate torque, speed, and current margins.

Should I compare required torque with stall torque?

No. Stall torque is not a safe continuous target. Compare the result with rated or continuous torque when possible. Use stall torque only to understand short peak limits.

Why is the safety factor included?

The safety factor covers starting load, surface changes, bumps, heat, and data sheet uncertainty. Higher values are safer for ramps, outdoor wheels, and payload changes.

What rolling coefficient should I use?

Use a low value for smooth floors and hard wheels. Use a higher value for carpet, rubber tires, grass, rough paths, or soft surfaces.

How does wheel size affect motor torque?

Larger wheels need more torque for the same force. Smaller wheels reduce torque demand, but they may handle obstacles poorly and may spin faster.

How does gear ratio affect the result?

A higher gear ratio reduces required motor shaft torque. It also increases required motor rpm. The motor must meet both torque and speed needs.

Why does current matter in motor selection?

Current affects driver size, wiring, battery load, heat, and runtime. A motor may meet torque needs but still require too much current for the system.

Can this replace physical testing?

No. It gives a strong planning estimate. Real testing is still needed because tire grip, gearbox losses, battery sag, and surface conditions can change performance.