Free Flat Rack Loading Calculator

What is a flat rack container?

A flat rack container is an open-sided ISO container designed for cargo that doesn't fit inside a standard dry van. Instead of four walls and a roof, it has just a reinforced deck, two collapsible or fixed end walls, and a set of lashing points welded along the side rails. Everything above the deck is open air, which is how you end up loading things like transformers, crawler excavators, yachts, and 11-meter pipe bundles on a container vessel in the first place.

Two sizes dominate the trade. The 20' flat rack has a usable deck roughly 5,940 mm long by 2,394 mm wide, with a payload ceiling of about 31,000 kg for standard units and up to 45,000 kg for heavy-duty variants. The 40' flat rack stretches to about 12,080 mm by 2,438 mm of deck with a payload of around 40,000 kg. Both types come in fold-down end designs — the walls collapse flat when the rack is empty so carriers can stack them four-high for the backhaul — and fixed-end variants that are more rigid but less space efficient on the return leg. Which one shows up at your port depends entirely on what the carrier has free. You usually don't get to pick.

Why a flat rack loading calculator?

Loading a flat rack isn't like packing a box. With a dry van you're playing Tetris inside a rectangular room, and the constraints are volume and weight. With a flat rack you're working with open faces on four sides. That changes the whole problem. Now you have to think about how far your cargo can legally hang over each edge, whether the load's center of gravity falls inside the container footprint, which lashing points your rigging plan actually uses, and whether the next carrier on the route will accept the declaration at all.

A calculator beats a spreadsheet and a hand sketch because it catches the silent failures. A 14.2 m piece of equipment will fit on a 40' flat rack — the deck is 12.08 m, so you have 2.12 m of overhang to distribute — but if you don't symmetrically split that overhang between the front and rear, the longitudinal center of gravity walks forward of the ideal position and the carrier rejects your booking at the port. A calculator renders that in real time. A spreadsheet tells you three days later.

The other thing a calculator gives you is a document. When you're negotiating an OOG surcharge with three carriers, none of them care about your verbal description of the load. They want a dimensioned plan with overhang labeled, weight labeled, and the cargo visualized on the specific container type they operate. Stow8 exports that as a one-page PDF you can attach directly to the booking request.

Overhang limits you can't eyeball

Carrier rules vary by lane, vessel, and operator, but there is an industry floor that every shipper should internalize. Front and rear overhang — what the trade calls longitudinal overhang — is usually accepted up to around 500 mm per end before it becomes a noteworthy out-of-gauge charge. Beyond 1,000 mm you're in the territory where the carrier asks for a lashing plan and a pre-approval. Beyond 2,000 mm you're talking about custom rigging and, on some routes, escort vehicles for the road leg.

Side overhang is the one that catches newcomers. Unlike front and rear, side overhang is measured from the outside edge of the container side rail, not the deck. Up to roughly 100 mm of side overhang is routine. Past that you're into OOG territory and a surcharge. Past 300 mm per side you usually need to book a platform container instead of a flat rack, because the truck envelope can't legally carry that width on a flat rack chassis in most jurisdictions.

Overhead overhang is the forgotten dimension. Anything that sticks above the top rail of the flat rack counts as overhead overhang, and it matters because the cargo hold of a container vessel only has so much vertical clearance per stack. A piece of cargo that's 2.8 m tall on a 2.04 m flat rack gives you 760 mm of overhead overhang, which means the slot above that flat rack is unusable. That's two slots worth of freight from one booking, which is why the carrier charges you for both. Stow8 surfaces all three overhang dimensions — longitudinal, transverse, and overhead — in the plan summary so nothing gets missed.

The carriers are not consistent on thresholds. MSC, Maersk, CMA CGM, Hapag-Lloyd and ONE each publish their own OOG classification tables and they disagree on the edge cases. Rather than try to match every table, Stow8 flags anything past the generic industry floor so you know to check. The goal is to never be surprised at the port.

Center of gravity: the thing that stalls your shipment

A flat rack with 18 tons of machinery loaded 80 cm off-center is a shipment that doesn't move. Vessel stowage planners care about two axes. Longitudinal center of gravity — the balance point along the length of the container — affects truck stability on the road leg, crane balance during the ship-to-shore lift, and vessel trim once stacked in the hold. Carriers will generally accept a longitudinal CoG offset up to about 10 percent of the container length, which is roughly 600 mm on a 20' flat rack and 1,200 mm on a 40'. Past that, the carrier needs a stowage exception memo and your booking goes on hold.

Transverse center of gravity — the balance point across the width — is the one that actually overturns equipment. The rule of thumb is that the transverse CoG has to fall within about 5 percent of the container width, which is barely 120 mm on a 2.44 m deck. Eccentric loading causes lateral listing during seaway motion, which means the neighboring stack also moves, which is why port operators are strict about it. Stow8 computes both the longitudinal and transverse CoG automatically as you drag items onto the flat rack, and surfaces them with a green marker in the 3D view. If the marker goes red, you know you have to reposition something before you export the plan.

How Stow8 helps

• 3D visualization — See exactly how every piece fits on the flat rack deck before the truck leaves the yard. Rotate, zoom, inspect from every angle.
• Automatic CoG marker — The green dot shows your load's longitudinal and transverse center of gravity in real time. It turns red when you violate carrier tolerances.
• Overhang callouts — Front, rear, left, right, and overhead overhang are highlighted as you position items. Symmetric and asymmetric distributions are labeled so you know what to tell the stevedores.
• OOG surfacing — Any out-of-gauge condition surfaces in the plan summary with the exact dimension that triggered it. No surprises on the booking request.
• PDF export — Download a printable loading plan with dimensions, CoG, container type, and a 3D rendering. One page, ready to hand to your port operator.
• Excel import — Paste your item list from a spreadsheet. Names, dimensions, weight, quantity, stackability — all come in with one click.

Common flat rack use cases

• Oversized machinery — transformers, turbines, industrial HVAC units, generators, compressors
• Construction equipment — excavators, wheel loaders, concrete mixers, asphalt pavers, cherry pickers
• Pipe bundles and steel sections — pipe in standard 12 m lengths, I-beams, rebar bundles, plate stacks
• Yachts and boats — hulls up to around 12 m, tender boats, racing sculls, rigid inflatable boats
• Pre-fab wall panels and modular building components — containerized housing, bathroom pods, curtain wall sections
• Mining equipment — rock drills, crushers, conveyor sections, undercarriages with track assemblies
• Windmill and renewable energy components — nacelle housings, gearbox assemblies, tower sections under 12 m
• Military and heavy defense cargo — armored vehicles, radar assemblies, field equipment palletized for sea transit

Flat rack vs open top vs platform container

The three open-faced container types look similar on a booking screen but they serve very different problems. Flat rack containers have two collapsible end walls and an open top, so cargo can be taller than the container height. Open top containers keep all four walls but lose the roof, which is useful for top-loaded cargo that has to drop in with a crane but still fits within the container's internal width. Platform containers are just a deck with no walls at all, used for cargo that exceeds the container length as well as the width — heavy rail sections, ship propellers, wind tower bases. If your cargo has a single dimension that overflows the standard ISO envelope, a flat rack is usually the right choice. Two overflowing dimensions usually means a platform. Stow8 supports all three and will default-suggest the right container type based on the item footprint.

Frequently asked questions

What dimensions does a 40' flat rack fit?

Internal usable deck is roughly 12,080 mm long by 2,440 mm wide, with a max payload of about 40,000 kg on standard flat racks and 45,000 kg on heavy-duty. Usable height from the deck to the container top rail is around 2,040 mm. Outside of that, you're into OOG territory.

Can I load cargo wider than 2.44 m on a flat rack?

Yes — that's actually one of the key flat rack use cases. Side overhang classifies the shipment as OOG and triggers a surcharge, but it's routine for cargo to extend up to the full transport-vehicle width, which is typically 2.55 to 3.2 m depending on the road and rail jurisdiction.

How is a flat rack loading calculator different from a regular container packing calculator?

Most container calculators assume closed walls and care only about volume utilization — how many boxes fit in the box. Flat rack loading has no walls, so the problem changes. You have to model overhang on all four sides plus overhead, you have to surface center of gravity in real time, and you have to think about lashing points and OOG surcharges. Stow8 is built for this problem specifically.

Does the calculator export to PDF?

Yes. Every plan exports to a one-page PDF with the 3D view, container dimensions, item list, overhang callouts, center of gravity, and a recommendation line. It's designed to be handed directly to a port operator or attached to a carrier booking request.

Do I need to pay?

No. Stow8 is free for the first 15 optimizations per rolling 30-day period. No credit card required. Upgrade only when you need more capacity — $18 per month for the Starter plan with 100 optimizations, $38 per month for the Pro plan with 400 optimizations and advanced features.