How to Get Units of g: Centrifugation Guide

This guide explains how to convert relative centrifugal force (RCF), commonly expressed as “g”, from revolutions per minute (RPM) and rotor radius. You’ll learn the formula, how to use it manually or with a calculator, and common pitfalls to avoid.

Formula

The standard formula to calculate RCF (in units of g) is:

RCF = 1.118 × 10^-5 × r × (RPM)²

Where:

• RCF = relative centrifugal force in units of g (e.g., 1000 g = 1000 times Earth’s gravity)
• r = radius of the rotor in centimeters (cm) – measured from the center of the rotor to the bottom of the tube when spinning
• RPM = revolutions per minute of the rotor

If you know the desired g-force and want to find the required RPM, rearrange:

RPM = sqrt( RCF / (1.118 × 10^-5 × r) )

Worked Example

Using the calculator defaults: RPM = 3000, radius = 10 cm.

Step 1: Square the RPM: 3000² = 9,000,000

Step 2: Multiply by the radius: 9,000,000 × 10 = 90,000,000

Step 3: Multiply by the constant 1.118 × 10^-5 (which is 0.00001118): 90,000,000 × 0.00001118 = 1006.2

Rounded to the nearest whole number, RCF ≈ 1006 g.

When to Use This

Use this conversion whenever you need to set a centrifuge to a specific g-force, for example in cell culture, DNA isolation, or protein precipitation. Many protocols specify g-force rather than RPM because g-force is independent of rotor size. If your centrifuge only displays RPM, you must know the rotor radius to convert.

How to Read the Result

The result is a dimensionless number representing multiples of Earth’s gravity. For instance, 1000 g means the sample experiences a force 1000 times stronger than gravity. Higher g-forces sediment particles faster, but may damage delicate cells or cause excessive heat.

Common Mistakes

• Using the wrong radius: The radius is not the rotor’s diameter, nor the distance to the top of the tube. It’s the distance from the rotor center to the bottom of the tube when the tube is in the rotor at rest.
• Mixing units: The formula requires radius in centimeters. If your rotor radius is in inches, convert to cm (1 inch = 2.54 cm) before using the formula.
• Forgetting to square the RPM: The RPM is squared in the formula. A common error is to multiply RPM by radius directly.
• Rounding too early: Use the full precision during calculation, then round only at the final answer.

Tips

• Always check your centrifuge manual or rotor label for the correct radius. Some rotors have a “max radius” and a “min radius” – use the one that matches your tube position.
• If you’re using a swinging-bucket rotor, the radius changes slightly during spinning due to the bucket swinging out. Use the radius at full swing for accuracy.
• For quick conversions, many online calculators and apps exist, but understanding the formula helps you verify results.
• When comparing protocols, note whether they give g-force or RPM. If they give RPM, they usually specify the rotor type – you must use the same rotor to get the same g-force.

Troubleshooting

My result seems too high or too low. Recheck the radius. A typical error is using the rotor diameter instead of the radius. Also verify that your RPM is correct – some centrifuges display speed in rpm, but others in rcf (g). If your machine shows g directly, you don’t need this conversion.

I don’t know the radius of my rotor. Look for a label on the rotor itself or in the centrifuge manual. If you can’t find it, measure the distance from the center of the rotor to the bottom of a tube placed in the rotor. For angle rotors, measure to the outermost point of the tube.

My centrifuge only shows g, but I need RPM for a protocol. Use the inverse formula: RPM = sqrt( g / (1.118e-5 × r) ). Enter the g value and radius, and the calculator will give you the RPM.

I’m getting different results from different online calculators. Some calculators use a slightly different constant (e.g., 1.12e-5) or assume a different radius definition. Stick to one reliable source and always specify the radius you used.

Frequently Asked Questions

Q: What is the difference between g and RPM?
A: g (RCF) is a measure of the force applied to the sample, while RPM is the rotational speed. The same RPM can produce different g-forces depending on the rotor radius.

Q: Why do some protocols say “centrifuge at 10,000 g” instead of RPM?
A: Because g-force is a standardized measure that is independent of the centrifuge model. RPM alone is meaningless without knowing the rotor radius.

Q: Can I use this formula for any centrifuge?
A: Yes, as long as you know the rotor radius. The formula is universal for any centrifuge.

Q: What if my rotor radius is given in millimeters?
A: Convert to centimeters by dividing by 10 (e.g., 150 mm = 15 cm).

Q: How do I find the radius of a fixed-angle rotor?
A: Measure from the center of the rotor to the bottom of the tube when it is in the rotor at its maximum angle. The manual may also list the “max radius” in cm.

Q: Is it safe to exceed the maximum g-force of my centrifuge?
A: No. Always stay within the rotor’s rated maximum speed (RPM) and g-force. Exceeding it can cause rotor failure and serious injury.

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in rpm | Rotor speed | RPM | 3000 | 0
in r | Rotor radius | cm | 10 | 0
out g | Relative centrifugal force | g | (rpm^2 * r * 1.118e-5) | 0
out rpm | Equivalent RPM | RPM | sqrt(g / (r * 1.118e-5)) | 0