In reading about warships of the early modern era, it's difficult to compare sizes because older ships tend to be measured in tons (or tuns) of burden (or burthen), compared to modern vessels being measured in tons (or tonnes) of displacement.
The former is a measurement of volume, the latter of weight, but for a floating vessel they should in theory be equivalent. Except that burden may be calculated by the simple dimensions of a ship, not its actual volume.
Searching Wikipedia for famous ships, I've found a few listing both burden and displacement:
Ship Burden Displacement Ratio ---------------- ------ ------------ ----- Santa Maria 108 165 tons 1.53 HMS Victory 2,142 3,500 tons 1.63 USS Constitution 1,576 2,200 tons 1.40 HMS Orlando 3,740 5,493 tons 1.47 HMS Mersey 3,733 5,643 tons 1.51 Mayflower II 180 242 tons 1.34
These are fairly consistent; if the answer is "1.5 is usually fine," I can live with that. However, I have not yet found an example for a carrack (the Vasa is listed with displacement but not burden; the Mars either has a typo in one measure or the other, or uses a different measurement system). Being relatively heavily-built compared to these others, should I estimate the displacement of the Henry Grace à Dieu (1,000 tons burden) as, say, 1,700 tons?