How do I calculate the servo torque needed for a robot arm?

Torque in kg·cm = (payload in kg × arm length in cm) + (arm weight in kg × half the arm length in cm), multiplied by a safety factor. The worst case is the arm held horizontal.

Is an SG90 servo strong enough for a robot arm?

An SG90 provides about 1.8 kg·cm — enough for a very light gripper or a short arm segment lifting a few tens of grams. Shoulder joints on even a small desktop arm usually need 10 kg·cm class servos like the MG996R or better.

Which joint of a robot arm needs the strongest servo?

The shoulder — it must lift every downstream segment, every downstream servo, and the payload, all at the longest lever arm. Calculate from the gripper backwards and the shoulder requirement grows fast.

Servo Torque Calculator for Robot Arms

How this calculator works

Torque is force times distance. With the arm horizontal (worst case), gravity acts on the payload at the full arm length and on the arm's own mass at roughly its mid-point:

Torque (kg·cm) = [payload(kg) × L(cm)] + [arm weight(kg) × L(cm) ÷ 2]
Required spec = torque × safety factor

Working in kg and cm gives the answer directly in kg·cm — the unit hobby servos are sold in — with no conversion.

Multi-joint arms: calculate from the gripper backwards. Each joint's "payload" includes everything past it — segments, servos, wires and the object. The worked 3-joint example walks through a complete arm.

Servo classes at a glance

Class	Typical torque	Example	Good for
Micro (9 g)	1.5–2.5 kg·cm	SG90 / MG90S	Grippers, sensors, tiny arms
Standard	8–12 kg·cm	MG996R	Small arm elbows/shoulders
High-torque digital	18–25 kg·cm	DS3218 / DS3225	Desktop arm shoulders
Serial bus / robot servos	25–60+ kg·cm	Dynamixel, LX-16A class	Serious arms, hexapods

Common questions

Why does my servo work vertically but fail horizontally?

Torque demand is proportional to the cosine of the angle from horizontal — it's maximum with the arm level and near zero pointing straight up or down. Always size for horizontal.

Metal or plastic gears?

For anything that can be back-driven by a falling arm, metal gears. A stripped plastic gear train is the classic first-arm failure.

Parts this calculation leads to

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High-Torque Servos

20 kg·cm+ digital servos for shoulder joints.

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Standard Servos

MG996R-class workhorses for mid joints.

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Servo Brackets

Aluminium brackets and horns for arm frames.

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Servo Drivers

PCA9685 boards + a proper 5–7 V supply.

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