Ask when FCL beats LCL and you will be told 13 to 15 cubic metres. That figure comes from particular lanes at a particular time, and treating it as a rule has cost shippers real money in both directions — booking a container at 12 cubic metres on a lane where the crossover was 20, or paying groupage charges at 16 on a lane where it was 9.
FCL vs LCL Cost Comparator computes the crossover from your own numbers. It bills LCL correctly on the revenue ton — the greater of cubic metres and tonnes, with your minimum applied — adds the origin and destination charges that make LCL quotes unpredictable, and compares that against the flat cost of the containers your cargo actually needs, checked against both volume and payload. The result is a break-even in cubic metres for your lane, your rates and your cargo density, plotted as a cost curve so you can see how far you are from the tipping point.
It also prices the things that decide the question when freight is close. Extra transit days on LCL are working capital sitting in a warehouse, and for high-value goods that carrying cost regularly exceeds the freight saving. FCL carries demurrage and detention exposure that LCL does not. Both are included as explicit, adjustable lines rather than left as footnotes, so the recommendation reflects total cost rather than the ocean rate alone.