What’s the maximum load a forklift can handle?

Introduction – why “maximum load” is not a single number
The question “What’s the maximum load a forklift can handle?” sounds simple, but the answer is a moving target that depends on at least twelve inter-linked variables. A truck rated 5 000 kg on the data plate may legally lift only 2 200 kg in the real world once you insert a 600 mm load centre, lift to 6 m, tilt forward 5° and drive on a 6 % cross-slope. Understanding how manufacturers derive those limits—and how they erode in daily operation—is the difference between safe productivity, expensive downtime, or a catastrophic tip-over.

The physics in one page
A forklift is a counter-balanced lever. The front axle acts as the fulcrum.

Load moment = weight (kg) × horizontal distance from fulcrum (mm)

Truck moment = counterweight mass × its horizontal distance from fulcrum

Stability margin = truck moment − load moment (must be ≥ 0 per ISO 3691-1)
Because the mast moves the load both vertically and forward, the moment arm grows with lift height and tilt angle. A 1 000 kg pallet at 500 mm load centre exerts the same moment as 500 kg at 1 000 mm—proof that “capacity” is meaningless without stating geometry.

Name-plate capacity – the only number that counts in court
ISO 3691-1 requires every forklift to carry a capacity plate showing:

Rated capacity (Q) in kg

Load centre distance (c) in mm

Mast type (standard, duplex, triplex, quad)

Lift height (h) at which Q is valid

Down-rated capacity graph for higher lifts or attachments
Any configuration not on the plate is illegal to lift. Plates are calculated with the mast vertical, truck level, hard smooth floor, no attachment, no side slope, no wind. Change one parameter and the plate is no longer valid—an attachment must be followed by a new plate issued by the OEM or an accredited engineer.

Load centre: the silent killer
Standard industrial pallets (1 200 × 1 000 mm) assume a 500 mm load centre. Switch to 1 200 × 1 200 mm bulk bags and the centre moves to 600 mm. Moment rises 20 %, so a 4 t truck may drop to 3.2 t. Long-unwound coils or pipes can push the centre to 800 mm, cutting capacity almost in half. Always measure the actual centre of mass, not the middle of the pallet.

Lift height: capacity fades with every metre
Mast profiles deflect forward under load. A 5 t capacity at 3 m can fall to 3.8 t at 6 m because the load’s horizontal overhang increases 80–100 mm. Quad masts with telescopic rails suffer the most; a 15 m container mast may lose 35 % of base capacity. Use the OEM “height derate curve” never linear extrapolation.

Tilt angle – forward 1° ≈ 3 % capacity loss
Tilt is needed to pick up a pallet, but each forward degree adds 10–15 mm to the moment arm. Mast builders allow 3° forward tilt in the rating calculation; beyond that you must derate. Back-tilt does not restore capacity—it only stabilises while travelling.

Attachments – why sideshifters steal 10–20 %
Every attachment moves the load forward. A typical 2.5 kg sideshifter frame is 180 mm thick, so a 500 mm load centre becomes 680 mm. Derate = 680/500 = 1.36 → 26 % capacity loss. Multi-pallet clamps, rotators, fork positioners, layer pickers all publish their own “lost load” figure. The new rating must be re-calculated and stamped by a qualified engineer; simply fitting a larger counterweight is not acceptable unless the truck receives a new CE/UL certificate.

Tyres, ground speed and floor conditions

Cushion (solid) tyres on smooth concrete give the highest ratings; pneumatic tyres on uneven black-top can cut 5–8 % because the truck rocks.

Slope: ISO 3691 allows 3.5 % longitudinal and 1.5 % lateral slope in the rating test. A 5 % cross-slope can reduce capacity 15 %.

Speed: high-mast trucks may have speed-related derates above 12 km/h because dynamic mast sway increases load swing.

Ground clearance: too much (≥ 200 mm) shortens the wheel-base and reduces stability—common in rough-terrain forklifts.

Counterweight strategies – bolt-on, slab, liquid, battery
Electric trucks use 1 500–2 000 kg of lead-acid or LiFePO₄ pack as movable ballast. Adding extra steel plates is possible, but any > 5 % mass change needs new stability tests. Diesel trucks sometimes carry bolt-on rear slab weights; again, a new plate and new brake-steering calculations are mandatory. Never weld scrap steel to the counterweight—fatigue cracks appear within 500 h.

Dynamic events – braking, cornering, tipping
ISO 22915 series requires a truck to pass 3 500 tip tests:

Longitudinal: 1.5 × rated load on forks, truck on 4 % slope, brakes applied, no tip.

Lateral: 0.6 × rated load, forks 300 mm off centre, 0.3 m/s cornering, 0.4 g lateral acc., no tip.
These are lab limits; real-world operators can exceed 0.5 g while turning on a dock plate. Fleet managers should fit inertia loggers and alarm at 0.35 g.

Hydraulic limits – cylinders, hoses, relief valves
Capacity is also the lowest of three mechanical ceilings:

Stability moment (already discussed)

Hydraulic lift capacity (cylinder bore × relief pressure)

Fork and carriage strength (ISO 2330 forks, ISO 6055 carriage)
Example: a 5 t truck may reach 5 t hydraulically at 3 m, but at 7 m the same cylinder pressure can only raise 4 t because of increased piston side-load friction. Relief valves must be sealed and tamper-proof; field re-adjustment to “get a bit more” is illegal and invalidates insurance.

Chain safety factor – 5:1 is not 5 × overload
Lift chains are rated at 5 × static proof load, but dynamic loading, wear, corrosion and side bending reduce life exponentially. A 10 % loss of diameter (1.5 mm on 15 mm chain) equals 27 % strength loss. Inspect every 40 h with go/no-go gauges; replace both chains as a matched pair to prevent unequal loading.

Fork wear – 3 % loss = 9 % capacity drop
ISO 5057 allows maximum 10 % wear on fork heel thickness; beyond that the blade acts like a knife edge and can snap. A 5 t fork worn 3 % already loses 9 % capacity because the section modulus is cubed. Calipers and magnetic templates are <$30; use them monthly.

Battery electrics vs. diesel – same capacity, different ballast
A 5 t electric needs ~1 250 Ah @ 80 V (8.5 kWh usable) to match a diesel counterweight. Li-ion packs are 30 % lighter than lead-acid, so OEMs either lengthen the wheel-base or add steel slabs. Retrofitting Li-ion into a lead-acid truck without re-certifying the plate is prohibited.

Rough-terrain / telehandler crossover
RT forklifts (ISO 10896) use oscillating axles, 4-wheel steer and 40 % gradeability. Their “capacity” is given as a graph over height and reach, e.g. 5 t at 2 m radius, 1.8 t at 4 m forward reach. The same machine may list 8 t on the plate—valid only when boom is fully retracted and lowered. Always read the load chart on the dashboard before every pick.

Automated trucks – capacity derate for sensors
AGV forklifts carry extra 100–150 kg of LiDAR, cameras and steel guard plates high on the mast, raising the centre of gravity 80–120 mm. OEMs typically derate 5 % to keep stability margin identical to manual version. Retro-fitting autonomy to an existing truck therefore requires a new capacity plate.

Standards summary – a 30-second cheat sheet

ISO 3691-1: industrial counterbalance safety

ISO 22915-1 to 21: stability tests for every truck type

ISO 2330: fork technical specification

ISO 5057: fork wear inspection

ANSI/ITSDF B56.1 (US) – mirrors ISO but uses inch-pound units

EN 16307 (EU) – adds vibration and noise limits

UL 583 – electric battery safety (US)

Practical checklist – how to know today’s real limit

Read the plate – is the attachment shown?

Measure the load centre – tape rule from fork face to CG.

Check mast vertical – no forward tilt beyond 3°.

Verify lift height – use the derate curve if > 3 m.

Look at the floor – side slope ≤ 1.5 %?

Inspect forks – heel wear ≤ 10 %, no cracks.

Scan chain – 5 % elongation or 1.5 mm wear = replace.

Weigh the load – if unknown, use pallet scales.

Compare calculated moment with truck moment – if in doubt, lift 50 cm and creep; any rear-wheel lift means abort.

Log the result – fleet software can auto-derate next time.

Common myths – busted
Myth 1: “Add more counterweight, you can lift anything.”
Truth: Stability tests are dynamic; extra steel may pass static but fail lateral 0.4 g test.
Myth 2: “Back-tilt restores lost capacity.”
Truth: It only shifts the moment arm rearward while static; braking brings it back forward.
Myth 3: “Side-shift uses the same chart.”
Truth: Even an integral sideshifter moves the load 80–180 mm forward; plate must be re-issued.
Myth 4: “Forks rated 5 t will always take 5 t.”
Truth: Fork capacity is per pair, vertical load, no impact, no wear. A single worn fork can snap at 60 % rating.

Take-away – a one-sentence rule for the field
If any one of these five parameters changes—load centre, lift height, tilt angle, attachment, or ground slope—look up the new number on the OEM derate chart or treat the lift as unsafe; the “maximum” is only the lowest value that survives physics, law and insurance, never the biggest figure you once saw on a brochure.