This seems like a good time to update my ink capacity measurements. I really thought I'd posted these on FPN before, but a search didn't turn them up. I measure the capacity by filling the pen with water, then expel the water and measure it using a graduated syringe. I make no attempt to account for water remaining in the feed, so my numbers will be slightly lower than those calculated by weight. I also decided to round everything to the nearest 0.1 cc, since haggling over hundredths would be silly and possibly misleading.
0.4 cc - Pilot Vanishing Point (with included converter)
0.5 cc - Sheaffer Snorkel
0.6 cc - Schmidt K5 converter
0.7 cc - Sheaffer Imperial (touchdown filler)
0.8 cc - Esterbrook J
0.8 cc - Sheaffer squeeze converter
1.0 cc - Sheaffer Skrip cartridge *
1.0 cc - Sheaffer Triumph (Vacuum-Fil)
1.0 cc - TWSBI Diamond 530
1.1 cc - Bexley Corona
1.1 cc - Pelikan M200
1.1 cc - Lamy 2000
1.1 cc - Parker 51 (aerometric)
1.2 cc - TWSBI Vac 700 (single stroke fill)
1.6 cc - Parker 51 (vacumatic)
1.7 cc - Gate City Belmont
1.9 cc - Wality eyedropper (standard size)
2.0 cc - TWSBI Vac 700 (two stroke fill)
2.2 cc - Gate City New Postal Jr. (bulb-filler)
2.5 cc - Edison Glenmont (bulb-filler)
3.0 cc - Wality eyedropper (oversized)
6.4 cc - Varuna Gajendra (giant eyedropper)
* Sheaffer cartridges are supposed to come from the factory with 1 cc, but when refilling them I can get about 1.4 cc in.
It's interesting to note how many classic fountain pens cluster right around 1.0-1.1 cc capacity.
We must remember that the amount of writing you can get from a pen is not a simple function of how much ink it holds. A smaller nib tip, a drier nib and a more consistent feed can all stretch your writing. The Sheaffer Snorkel, for example, is known for having pretty good write-out, despite the small amount of ink it holds. It's the Prius of fountain pens!
The TWSBI Vac 700 is listed twice. Most vac-fill pens are intended to fill normally with a single stroke of the plunger. There is also a somewhat tricky two-stroke technique to fill the entire reservoir, and TWSBI sell a special ink bottle to make it easier and safer to perform without spilling ink. So, I decided to measure it both ways.
Also, more ink is not always better. Whenever go you above 2 cc, then you need to start worrying about ink surges which may overwhelm the feed, then your pen may burp up drops of ink onto the page. I never saw that happen with my New Postal Jr, but I have seen it a couple of times with the Edison Glenmont.