Servo Torque Math: How to Size Servos for Control Surfaces
Servo torque math starts with the load at the control horn and the length of the servo arm. A servo rated at 3.5 kg-cm at 6.0V holds about 48.6 oz-in of torque, which suits larger control surfaces. A micro servo rated at 1.6 kg-cm at 6.0V weighs only 5.8 g, which suits small planes with light surfaces.
Quick facts
| Fact | Value |
|---|---|
| 1250MG torque | 3.0 kg-cm at 4.8V, 3.5 kg-cm at 6.0V |
| 1250MG speed | 0.14 sec at 4.8V, 0.12 sec at 6.0V |
| 1250MG size | 31.0 x 16.3 x 29.5 mm, 25.0 g |
| DSM-44 torque | 1.6 kg-cm at 6.0V |
| DSM-44 speed | 0.07 sec per 60 degrees at 6.0V |
| DSM-44 size | 22 x 8.7 x 22 mm, 5.8 g |
How does servo torque math work?
Torque equals force times arm length, so a longer arm needs more torque for the same load.
A 1 kg-cm rating holds 1 kg at a 1 cm arm, by the definition of the unit.
A 3.5 kg-cm servo on a 2 cm arm holds 1.75 kg, from 3.5 divided by 2.
What torque does a control surface need?
Measure the load at the horn, then divide by the servo arm length in cm.
Size the servo above the measured load, so it never works at its stall rating.
The 1250MG gives 3.5 kg-cm at 6.0V, which covers larger surfaces with margin.
How do voltage and speed change the answer?
The 1250MG torque rises from 3.0 kg-cm at 4.8V to 3.5 kg-cm at 6.0V.
Its speed quickens from 0.14 sec at 4.8V to 0.12 sec at 6.0V.
The DSM-44 moves 60 degrees in 0.07 sec at 6.0V, which is the fastest figure here.
When does a standard servo suit the job?
The MG996R is a standard size metal gear servo for steering, retracts, and control surfaces.
Its metal gears handle loads that strip plastic-gear servos, per its listing.
The MG996R costs $15.99, and its listing states no torque figure, so size it by fit and gear type.
What did we check against the listings?
We matched the 3.5 kg-cm torque and the 48.6 oz-in figure to the 1250MG listing.
We matched the 1.6 kg-cm torque and the 0.07 sec speed to the DSM-44 listing.
We counted 1 image on each of the three servo listings.
How the options compare
| Servo | Torque at 6.0V | Speed | Weight | Price |
|---|---|---|---|---|
| 1250MG mini | 3.5 kg-cm | 0.12 sec | 25.0 g | $18.00 |
| DSM-44 micro | 1.6 kg-cm | 0.07 sec/60deg | 5.8 g | $22.00 |
| MG996R standard | Not stated | Not stated | Not stated | $15.99 |
How do you size a servo in 5 steps?
- Measure the load at the control horn in kg.
- Measure the servo arm length in cm from the output shaft.
- Divide the load by the arm length to get the torque in kg-cm.
- Pick a servo rated above that figure, with margin for flight loads.
- Check the speed and weight: 0.07 sec and 5.8 g on the DSM-44.
Parts that fit this guide
Check stock before you order, because in-stock parts ship from New York in 1-2 business days. Prices in this guide are the live store prices verified on November 12, 2026.
1250MG Metal Gear 25g Mini Servo ($18.00)
1250MG metal gear mini servo at $18.00. It gives 3.5 kg-cm at 6.0V in a 25 g case for control surfaces.
DSM-44 Digital Metal Gear Micro Servo ($22.00)
DSM-44 digital metal gear micro servo at $22.00. It gives 1.6 kg-cm at 6.0V and moves 60 degrees in 0.07 sec at 5.8 g.
MG996R Metal Gear Servo ($15.99)
MG996R metal gear servo at $15.99. It is a standard size servo with metal gears for steering, retracts, and control surfaces.
Shop the full range in the Servos category at Quality RC Parts. See also: Related guide.
Frequently asked questions
How do kg-cm and oz-in relate?
The 1250MG listing gives both: 3.5 kg-cm at 6.0V equals 48.6 oz-in.
Does higher voltage raise servo torque?
Yes, on the 1250MG. Its torque rises from 3.0 kg-cm at 4.8V to 3.5 kg-cm at 6.0V.
Which servo is fastest here?
The DSM-44 moves 60 degrees in 0.07 sec at 6.0V, which beats the 0.12 sec figure of the 1250MG.
What torque does the MG996R give?
The MG996R listing states no torque figure. Size it by its standard size and metal gears.
Sources
Last updated: November 14, 2026.