Pilot Tools

Center of Gravity Calculator

Weigh your plane at the wheels, enter the distances from the spinner, and see where your CG actually sits versus the target on your plans. Supports inches/mm, ounces/grams, and fractions like 3 1/4 .

About This Tool

There are several good ways to balance a model. This calculator uses the weigh-at-the-wheels method, the same one full-scale aircraft use. You set the plane on scales at its landing gear, measure how far each wheel is from the spinner, and the math works out where the center of gravity actually is.

Fingertip or stand balancing only tells you which way the plane is off. This method also tells you by how much (for example, 0.4 in tail heavy), so you know how far to move the battery or how much weight to add. It needs no lifting, so it works well for heavy, large, or awkward models that are hard to balance by hand.

New to this? Read the Weighing & Measuring Guide below before you start.

Other ways to balance, and when to use this one

Fingertip balance. Lift the plane by the wing at the CG marks and see whether the nose drops or the tail drops. Quick and free. Good for small and park-size models, but harder to judge on heavier planes, and it doesn't tell you how far off you are.
Balancing stand. The wing rests on two pointed supports set at the CG. More precise than fingertips and good for most sport models. It's still a balances / doesn't-balance check, so you adjust and re-check until it settles.
Hanging (plumb line). Hang the model from a point and let a plumb line show where the CG falls. Useful for gliders, flying wings, and odd shapes, but more setup.
Weigh at the wheels (this tool). Scales under the gear plus three tape-measure distances. Best when you want an actual number, when the plane is too big or heavy to lift and balance comfortably, or when you want to save a record and compare after changes like a new battery, motor, or engine. Needs landing gear, so it doesn't suit hand-launch gliders or belly landers.

What this tool doesn't do

Choose your CG for you. The target (A → C) comes from your plans or manual. With no plans, a common starting point for a conventional wing is about 25–30% of the wing chord back from the leading edge, but treat that as a starting point, not a spec.
Check side-to-side balance. This is front-to-back (fore/aft) balance only. Check lateral balance separately by supporting the plane at the spinner and the tail and seeing whether a wing drops.
Account for fuel. Glow and gas models are normally balanced with the tank empty. Electrics balance with the flight battery installed where it will fly.

Use at your own risk

This tool is provided as a convenience only. The Lake Sawyer Hawks RC Club is not liable or responsible for the results it produces or for any damage or injury resulting from its use. Always physically verify your aircraft's balance against the manufacturer's or plan's specifications before flight.

Calculator

Aircraft Type

Length Unit

Weight Unit

A = spinner · B = main wheels · D = tail wheel · C = target CG · ● = your CG

A → B distance ( in )

A → D distance ( in )

A → C target CG ( in )

Main wheels weight ( oz )

Tail wheel weight ( oz )

Fwd tolerance ( in )

Aft tolerance ( in )

Fractions welcome in any field: 3 1/4, 5/8, or plain decimals.

Enter your measurements to see results.

Weighing & Measuring Guide

Equipment

Digital scale(s) — one is sufficient (see Method 3 below); two or three make the job faster. Kitchen or postal scales suit most sport models; a bathroom scale handles giant scale. Example scales
Rigid board or straightedge — spans both main wheels across a single scale (Method 2).
Support blocks — matched to the height of the scale platform, used to substitute for a second scale and to hold the aircraft in level flight attitude.
Tape measure or ruler — for the A → B, A → D, and A → C distances.
Manufacturer's plans or manual — for the recommended CG location and acceptable range.

Weighing setups

Any of the setups below produces the same result. Whichever you use, keep the fuselage reference line level (dashed line) for every reading — raise the lower scale or block until it is. Tricycle gear works the same way: D is the nose wheel instead of the tail wheel, and since most trikes already sit close to level, you usually won't need a riser.

Method 1 — Two scales. One scale under the main wheels (B), a second under the tail or nose wheel (D). Raise the lower scale on a block until the fuselage is level, then record both readings.

Method 2 — Single scale under the main gear. A rigid board spans both main wheels onto one scale, giving the full main-gear weight (B) in one reading. Zero (tare) the scale with the board in place, or subtract the board's weight.

Method 3, Step 1 — One scale, main wheels first. Scale under the mains (B); a block as tall as the scale sitting on its riser supports the tail, so the fuselage stays level. Record the main-wheel reading.

Method 3, Step 2 — Swap and weigh the tail. Put a block the height of the scale under the mains, and move the scale onto the riser under the tail wheel (D). The matched heights keep the attitude identical between the two readings.

Procedure

Prepare the aircraft in ready-to-fly condition. Flight battery installed in its normal position; hatches, cowl, and canopy in place.
Level the aircraft in flight attitude. Raise a taildragger's tail until the fuselage reference line is level and support it there — an aircraft weighed sitting tail-low will not balance correctly. Verify with a small spirit level.
Weigh at the wheels. Record the main-wheel weight (B) and the tail- or nose-wheel weight (D) using one of the setups shown above, keeping the aircraft level for every reading.
Measure the horizontal distances from the spinner (A). A → B to the main-wheel contact point and A → D to the other wheel's contact point, measured horizontally along the fuselage — never diagonally down to the ground.
Determine the target CG (C) from your plans. If the plans reference the wing's leading edge, convert it: A → C = spinner-to-leading-edge distance + specified distance behind the leading edge.
Enter the values and review the result. The red dot marks your measured CG, the orange dashed line the target, and the green band your tolerance. Adjust battery position or ballast, re-weigh, and repeat until the dot sits within the band — then confirm the balance physically before flight.

The calculation is the standard moment-balance equation: the CG is the weight-averaged position of the two support points. The tolerance band defaults to ±1/4 inch — adjust it to match the CG range on your plans.