The T class were the designed as replacements for the O, P and R class boats. Their size (1325 tons) was kept down to allow more hulls to be built within the tonnage restrictions of the London Treaty. In common with the S class boats they had a very powerful torpedo armament, carried all their fuel internally and were constantly being improved throughout the war. Boats specifically intended for the far east had extra fuel tanks fitted to increase the range.

The specifications as given above is for the first group.
The first T class submarine to be completed, HMS Triton had a slightly different displacement; 1095 / 1579 BRT.
At least two boats from the first group, HMS Tigris and HMS Torbay, had an external stern torpedo tube fitted giving them 11 torpedo tubes. They also carried an extra torpedo.

Further specifications for the later groups;
2nd Group;
Same as the first group except;
Complement: 61 men
Armament
1 4" gun
3 MG AA
11 21" torpedo tubes (6 bow, 2 bow external, 2 amidships external firing aft, 1 stern external)
17 torpedoes

3th Group;
Same as the first group except;
Complement: 61 men
Armament:
1 4" gun
1 20mm AA
3 MG AA
11 21" torpedo tubes (6 bow, 2 bow external, 2 amidships external firing aft), 1 stern external
17 torpedoes

Unidentified T-class submarine in the Irish Sea. Photograph taken by Charles James Sadler, RCNVR. First Class Stoker, Official number V-4963.

T-class submarines that have detailed patrol info added are shown in bold in the list below.