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CG Shift Calculator (Weight to Move or Add to Bring the Centre of Gravity into Limits)

Bring an aircraft's centre of gravity back inside its limits: how much weight to move between two stations to shift the CG a given distance, how much ballast to add or remove at a station, and the new CG after changing fuel or passengers, using the weight-shift and weight-change formulas.

Total weight

Current CG

Target CG (inside the envelope)

Station to move weight FROM (arm)

Station to move weight TO (arm)

Ballast station (arm) if adding/removing weight instead

Maximum gross weight

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How it works

weight shift: weight moved ÷ total weight = ΔCG ÷ distance moved → w = W × ΔCG ÷ (arm_to − arm_from) · weight change: w = W × (CG_new − CG_old) ÷ (arm_ballast − CG_new) (+ add, − remove) · CG = Σ moments ÷ Σ weights

The centre of gravity is the weighted average of every item's arm, so fixing an out-of-limits CG is simple lever arithmetic. Moving weight inside the aircraft changes the moment without changing the total, and the CG moves by the weight moved times the distance, divided by the total weight. Adding or removing weight changes both the moment and the total, so the formula references the new CG. The calculator gives both answers and checks the ballast option against maximum gross weight.

Worked example

A 2,400 lb aeroplane with its CG at 47.8 in needs to be at 46.5 in (1.3 in forward). Moving baggage from the 95 in station to the 37 in front seats: 2,400 × 1.3 ÷ 58 ≈ 53.8 lb. Alternatively, adding ballast at 12 in: 2,400 × 1.3 ÷ 34.5 ≈ 90.4 lb, bringing it to about 2,490 lb — still 60 lb under a 2,550 lb gross weight.

Frequently asked questions

What is the weight-shift formula?

Weight moved ÷ total weight = change in CG ÷ distance moved. Rearranged, the weight to move is total weight × required CG change ÷ distance between the two stations. The total weight does not change, only the moment.

Why is an aft CG dangerous?

It reduces longitudinal stability and the nose-down authority of the elevator, lightens pitch forces so over-rotation is easy, and makes stall and spin recovery harder or impossible beyond the aft limit. A forward CG is safer but raises stall speed and can make the flare impossible at the limit.

Can I just carry ballast to fix the CG?

Yes if it is secured at an approved location and the extra weight stays within gross and the envelope — but moving existing load (baggage, passengers) is usually better because it costs no performance. Always recompute for landing after fuel burn.

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For training, planning and education only. The aircraft's approved flight manual (POH/AFM), current charts, NOTAMs, weather and regulations govern every real flight. Verify with your instructor and official sources.