No, your figures for new construction are too high. The ones I gave actually include all neutrals that made ships available to the allies, ie Norway, which alone lost over 1 million grt. Thus there was a net decrease in tonnage during the war, although that was offset by the vast amount of emergency programme ships that were completed in late 1918 and 1919. Initial capture of Central Powers tonnage was also less - about 280,000 grt.

For gains/losses by month for the British Empire, see [www.gwpda.org]

And for all nations' losses:
[www.gwpda.org]

Although you'll find some slight variation to those figures in later works.

Merchant fleet sizes (including sailing vessels)in 1913 were:

British Empire: 20.5 million grt
US: 5.8 million grt (nearly 30% of which sailing vessels!) This figure is excluding Great Lakes.
Norway: 2.5 million grt
France: 2.3 million grt
Italy: 1.7 million grt
Japan: 1.5 million grt
Netherlands: 1.3 million grt
Russia: 1 million grt
Germany: 5 million grt
Rest: approx. 6.5 million grt

Interesting to see the result of the emergency war programme (UK) and the US shipbuilding programme. In 1921 the fleet sizes were:

UK: 22 million grt
US 17 million grt (!!!)