Electric Motor Sizing Calculator

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Work out the torque, speed and power a motor must provide for a real load, then see which requirements a candidate motor meets and which still need checking.Learn more ▾Show less ▴
Motor sizing starts from the load, not from a catalog. This calculator derives the requirement from what you actually know: a torque figure, a mechanism's geometry and masses, or a pump or fan operating point. It returns steady torque and speed, shaft power, peak and RMS torque over a motion cycle, reflected inertia, and an optional electrical running-current estimate. Every result states its shaft location and its quality. “Calculated” means the stated mathematics is complete for its inputs; it does not mean a physical motor has been validated. Checks a real motor still needs, such as starting capability, thermal duty and drive compatibility, are listed as not checked rather than silently passed. Nothing is assumed on your behalf: no default efficiency, power factor, friction, reserve or derating is inserted. The tool runs entirely in your browser.
Motor shaft Add gearing Load / output shaft
Change the shaft reference: convert the entered values to the new shaft, or clear them?
%
Where do I get this?Equipment data sheet, manufacturer curve, measured or calculated pull, geometry, or the motion requirement. Do not take live electrical measurements or rig improvised suspended-load tests to obtain inputs.
°
Solid cylinder inertia / Thin ring inertia / Point mass at radius
kg/m³
Add gearing
Include starting and stopping
Motion cycle segments
TypeStart speedEnd speedDurationTorque
Negative mechanical power is braking or absorbed power — energy the mechanism returns, which the motor and drive must handle.
Estimate electrical input
V
Hz
Current estimates are labeled by measurement location. Through a controller, supply current and motor phase current are different quantities.
Shown from earlier valid inputs — the current entries are incomplete or invalid.
An unfinished number was kept in its original format — finish or correct it before the new format applies.
Shaft power
Starting, duty and motor capability are not yet checked.
    Assumptions and calculation
      Compare a real motor
      Nameplate data
      r/min
      r/min
      A
      V
      Hz
      Torque-speed points
      SpeedContinuous torque
      Curve conditions
      Candidate motor checks
      InfoNext useful input
      Nominal rating reference: A power-only reference from historical catalog series (kW per an ABB IE2 catalog page; hp per a Baldor catalog page). It suggests the next nominal size by power alone — it selects no product and checks no torque, speed or thermal capability.
      Scenarios
      CSV preview (machine format)
      The file uses decimal points and SI headers regardless of the page's number format, so machines can read it anywhere.
      
        
      
      
      
      
      
      
      
    
      
  
      

      

      
  
      
  
  
  
      

    
    

    
    
      

    
    
      
      
      
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      About This Tool
      
  
      This calculator turns a real load into a motor requirements sheet: torque, speed and shaft power at a declared shaft, peak and RMS demand over a motion cycle, reflected inertia, and an optional supply-side current estimate. It checks a candidate motor's data against those requirements and reports what is met, what is not, and what still lacks evidence.

It deliberately does not select products, approve motors, or certify safety functions. Every formula and boundary follows the archived engineering sources listed below.
      
  
      Sources:
    ABB Technical Guide No. 7 ·
    Kollmorgen (2025) — Servo overloads
  
      

      

      
  
      How to Use
      
  
        
    
      1. Pick the entry path that matches what you know: torque or power, a load to move, a pump or fan, or a motor to check.
        2. 
    
      2. Enter the values you have and press Calculate — nothing is assumed for you, and every unit is converted exactly.
        3. 
    
      3. Add gearing, starting and stopping, or the electrical estimate as your data allows — each section names its own inputs.
        4. 
    
      4. Compare a real motor's data in the candidate section: the table separates met, not met, and not checked — take that sheet to your supplier.
        5. 
  
      

      


      
  
      
    
      
      
      
    
    How to Use
  
      
  
        
    
      1. Pick the entry path that matches what you know: torque or power, a load to move, a pump or fan, or a motor to check.
        2. 
    
      2. Enter the values you have and press Calculate — nothing is assumed for you, and every unit is converted exactly.
        3. 
    
      3. Add gearing, starting and stopping, or the electrical estimate as your data allows — each section names its own inputs.
        4. 
  
      

      


      
  
      Methodology
      
  
      Shaft power is P = T·ω with exact unit factors from conventional definitions (NIST SP 811). Motion cycles integrate trapezoidal or triangular profiles; RMS torque uses the time-weighted root mean square over the full cycle, dwells included. Inertia reflects physically as J divided by the square of the gear ratio; efficiency belongs on the torque path, not inside inertia.

Pump power is ρ·g·Q·H (hydraulic) divided by the pump-only efficiency; fan power is Q·Δp with matching pressure and efficiency definitions.

Electrical estimates state their measurement location, and efficiency and power factor are entered, not assumed.
      
  
      Sources:
    NIST SP 811 (2016) — B.8 conversion factors ·
    ABB (2022) — Technical Guide No. 7 ·
    Kollmorgen (2017) — RMS torque
  
      

      

      
  
      Understanding Your Results
      
  
      Results separate what is calculated from what still needs evidence. The steady operating point (torque, speed, shaft power) is complete once its inputs are complete. Dynamic results such as peak torque, cycle RMS and braking energy describe the demand, not a motor's approval: RMS is a useful screen for suitable cyclic applications, not an unconditional thermal pass.

The candidate table uses four outcomes. “Meets entered requirement” and “does not meet” are numeric comparisons under matching conditions; “not checked” means required evidence is missing; “not applicable” rows are excluded rather than counted as passes. Electrical figures state their measurement location, because through a controller the supply current and the motor phase current differ.

A green shaft-power number is not a purchase decision. Starting behavior, thermal duty, drive compatibility and mechanical ratings remain with the manufacturer's data and your supplier.
      

      

      
  
      Practical Examples
      
  
      Quick path: a load needs 10 N·m at 1,500 r/min at the motor shaft. Shaft power = 10 × 1,500 × 2π/60 ≈ 1.571 kW (about 2.106 mechanical hp). No design reserve selected; starting, duty and motor capability are not yet checked.

Pump path: 10 m³/h of water (1,000 kg/m³) against 20 m total dynamic head gives about 545 W of hydraulic power. With a pump-only efficiency of 0.70 the pump shaft power is about 778 W; at 1,450 r/min that is roughly 5.1 N·m at the pump shaft. The motor's electrical input power is higher again, by the motor's own losses.
      

      

      
  
      Sizing tips
      
  
      • Size from torque and speed, not power alone: two motors with the same kW rating can differ widely in torque at the speed you need.
• Include the whole cycle: acceleration torque, dwell time and braking all change the RMS and peak demand that matter for selection.
• Reflect inertia physically (J divided by the square of the gear ratio) and add the rotor inertia from the candidate's data sheet. A gear ratio helps torque, but the motor still has to accelerate itself.
• Keep shaft power and electrical input power separate; the difference is the motor's losses, and current estimates depend on where they are measured.
• Treat duty codes, service factors and IE classes as nameplate metadata, and leave a written trail of every assumption for your supplier.
      

      

      
  
      Sources
      
  

      

      
  
      All calculations are performed locally in your browser. No data is sent to any server.
      

      





          
          
          
      
            
      
              
      Frequently Asked Questions
      
              
      
              
      
                Can I calculate motor size from weight alone?
                
      No. Weight alone fixes neither torque nor power. The requirement also depends on speed, geometry (drum radius or screw lead), friction, incline, acceleration and the transmission. The guided load forms ask for exactly those inputs and show which of them drive the result.
      
              
      
              
      
                Does 25% duty mean one-quarter the motor power?
                
      No. Intermittent duty changes the thermal picture, not the torque physics: the motor must still produce the full torque whenever it runs. For suitable cyclic applications an RMS-torque screen helps compare heating against a continuous limit, but cycle timing, cooling and the manufacturer's duty rules (IEC 60034-1 duty types S1–S10) decide what is actually allowed — a duty code is metadata here, never an approval.
      
              
      
              
      
                Can a high-power motor still have too little torque?
                
      Yes. Power is torque times speed, so the same kilowatt rating can mean very different torque at the speed you need. A high-speed motor may deliver its power with little torque and need a gearbox; starting and low-speed torque depend on the motor type and drive, not on the kW number.
      
              
      
              
      
                Is motor current the same as supply current?
                
      Not in general. Through a controller or drive, the current drawn from the supply and the current in the motor phases are different quantities. This tool's estimates are supply-side and say so; motor phase current needs manufacturer drive data.
      
              
      
              
      
                Does stepper holding torque apply while rotating?
                
      No. Holding torque is the powered standstill rating. Usable torque while rotating comes from the pullout torque-speed curve, which depends on the driver as well as the motor — the requirement must fall inside that curve at every operating speed.
      
              
      
              
      
                What does “calculated” mean in the results?
                
      That the stated mathematics is complete for its inputs — nothing more. It does not mean the physical model has been verified for your machine or that a motor is approved. “Estimated” marks values resting on a declared simplification, and “not available” marks values whose inputs are missing. A separate list names which inputs you entered, which came from manufacturer data and which were illustrative.
      
              
      
              
      
                Which units and number formats are supported?
                
      Metric and US customary units side by side — N·m, lbf·ft, ozf·in, kW, mechanical and metric horsepower, m³/h and US gpm among others — converted with exact factors from conventional definitions. Number entry follows your selected locale's decimal and grouping conventions strictly, and changing a unit converts the value rather than reinterpreting the digits.
      
              
      
              
      
                Does the calculator send my data anywhere?
                
      No. All calculations run in your browser; scenarios you save or export stay on your device. No account and no network connection are needed once the page has loaded.