A loading gauge is a diagram or physical structure that defines the maximum height and width of railway vehicles and their loads. The loading gauge is to ensure that rail vehicles can pass safely through tunnels and under bridges, and keep clear of platforms, trackside buildings and other structures. Classification systems vary between different countries, and loading gauges may vary across a network, even if the track gauge is uniform.

The term loading gauge can also be applied to the maximum size of road vehicles in relation to tunnels, overpasses and bridges, and doors into automobile repair shops, bus garages, filling stations, residential garages, multi-storey car parks and warehouses.

A related but separate gauge is the structure gauge, which sets limits to the extent that bridges, tunnels and other infrastructure can encroach on rail vehicles. The difference between these two gauges is called the clearance. The specified amount of clearance makes allowance for the oscillation of rail vehicles at speed.

The loading gauge governs the size of passenger carriages, goods wagons (freight cars) and shipping containers that can travel on the relevant section of railway track. It varies between rail systems around the world and can even vary within a single railway system.

Over time, there has been a trend towards less restrictive loading gauges and greater standardization of them. Some older systems and lines have had their structure gauges expanded by raising bridges, increasing the height and width of tunnels and making other necessary alterations. Containerisation, and a trend towards larger shipping containers, has led rail operators to increase loading and structure gauges to compete with road haulage.

The term "loading gauge" can also refer to a physical structure, sometimes using electronic detectors using light beams on an arm or gantry placed over the exit lines of goods yards or at the entry point to a restricted part of a network. The devices ensure that loads stacked on open or flat wagons stay within the height/shape limits of the line's bridges and tunnels, and prevent out-of-gauge rolling stock entering a stretch of line with a smaller loading gauge. Compliance with a loading gauge can be checked using a clearance car. In the past, they were simple wooden frames or physical feelers mounted on rolling stock. More recently, laser beams have been used.

The loading gauge is the maximum size of rolling stock. It is distinct from the minimum structure gauge, which sets limits to the size of bridges and tunnels on a rail line, allowing for engineering tolerances and the motion of rail vehicles. The difference between the two is called the clearance. The terms "dynamic envelope" or "kinematic envelope", which include factors such as suspension travel, overhang on curves (at both ends and middle) and lateral motion on the track, are sometimes used in place of loading gauge.^{[citation needed]}

Railway platform height is also a consideration for the loading gauge of passenger trains. Where the two are not directly compatible, stairs may be required, which will increase loading times. Where long carriages are used at a curved platform, there will be gaps between the platform and the carriage door, causing risk. Problems increase where trains of several different loading gauges and vehicle floor heights use (or even must pass through) the same platform.