The original predictions made by the Royal Navy were that somewhere between 30-45,000 men could be recovered from Dunkirk. This was based on several assumptions; that the German advance would continue at the same pace, that troops could not be taken off the beaches of Dunkirk in large numbers, and that the evacuation would be made with the ships already available to Dover Command.

The initial predictions were based on information provided to the Admiralty by the British Army. This suggested that the German Army would have captured Dunkirk within two days. To quote Vice Admiral Bertram Ramsay's report on Operation Dynamo:

The military situation was thought to have deteriorated so rapidly that the Vice Admiral was informed by the Admiralty that it was imperative for " Dynamo " to be implemented with the greatest vigour, with a view to lifting up to 45,000 of the B.E.F. within two days, at the end of which it was probable that evacuation would be terminated by enemy action.

As it happened, the German Army paused before attempting to reduce the Dunkirk pocket. The reasons for this are numerous; the threat of Allied forces to the south and west, the need to consolidate logistics, political manoeuvring by the Luftwaffe, and the actions of the French Army at Lille. This gave the RN over a week to complete the evacuation. Rather than ending on the 28th May as expected, it ended on the 4th June. This was a major factor in increasing the number of troops carried.

The predictions also assumed that troops could only be effectively withdrawn through Dunkirk harbour. Ships could come alongside here, and board troops quickly over gangplanks. However, Dunkirk harbour was cramped, and had limited capacity. The beaches provided a much larger area off which ships could moor and embark troops. Unfortunately, ships could not get close to the beach without running aground, and so the troops would have to be loaded by boat, a slow process with the few boats typically carried by warships. Fortunately, the mobilisation of the Admiralty Small Vessels Pool, and the distribution of naval small boats from other stations, provided a solution. These mobilisations meant that many more small craft were available to ferry troops from the beaches to ships waiting offshore. The work done by engineers ashore, who improvised piers from lorries, should also not be neglected. Taking troops off the beaches was still a slow process, but increased the number of troops that could be evacuated by roughly 30%.

Finally, the predictions assumed that Dynamo would use only resources already available to Dover Command. As it happened, the evacuation saw many more ships transferred to Ramsay's command, along with the small boats already covered. On the 28th May, two flotillas of minesweepers joined the evacuation, as did every spare destroyer available to Portsmouth and Western Approaches Command. There were also constant arrivals of troop transports and similar craft. There were also a number of additions from ashore, with assault boats and pontoons from the Royal Engineers helping to transport troops out to the ships offshore. There were also contributions from the French, who ran a parallel evacuation carrying troops to ports in France and the UK.