Model rocket engines come in several standard sizes so that whenever possible, engines of different total impulse and from different manufacturers may be used in the same rocket. The more common engine sizes are in blue.
| Size | Available Impulse Classes | Diameter | |
| mm (in) | Length | ||
| mm (in) | |||
| 10.5 x 38 | 1/4A, 1/2A | 10.5 (0.41) | 38 (1.50) |
| 10.5 x 47 | A | 10.5 (0.41) | 47 (1.85) |
| 10.5 x 89 | B | 10.5 (0.41) | 89 (3.50) |
| 13 x 45 (T mini engines) | 1/2A, A | 13 (0.5) | 45 (1.75) |
| 13 x 50 | B | 13 (0.5) | 50 (1.97) |
| 18 x 50 | C | 18 (0.69) | 50 (1.97) |
| 18 x 70 Standard | A, B, C, D, E | 18 (0.69) | 70 (2.75) |
| 18 x 77 | D | 18 (0.69) | 77 (3.03) |
| 21 x 95 | D, E | 21 (0.83) | 95 (3.74) |
| 24 x 101 | F | 24 (0.94) | 101 (3.98) |
| 24 x 124 | F | 24 (0.94) | 124 (4.88) |
| 24 x 144 | G | 24 (0.94) | 144 (5.67) |
| 24 x 177 | G | 24 (0.94) | 177 (6.97) |
| 24 x 70 | D, E, F | 24 (0.94) | 70 (2.75) |
| 24 x 89 | E | 24 (0.94) | 89 (3.50) |
| 27 x 114 | E | 27 (1.06) | 114 (4.49) |
| 27 x 152 | F | 27 (1.06) | 152 (5.98) |
| 29 x 124 | E, F, G | 29 (1.14) | 124 (4.88) |
| 29 x 152 | F | 29 (1.14) | 152 (5.98) |
| 29 x 206 | G | 29 (1.14) | 206 (8.11) |
| 29 x 238 | H | 29 (1.14) | 238 (9.37) |
| 29 x 291 | H | 29 (1.14) | 291 (11.46) |
| 29 x 85 | F | 29 (1.14) | 85 (3.35) |
| 29 x 95 | F | 29 (1.14) | 95 (3.74) |
| 29 x 98 | F | 29 (1.14) | 98 (3.86) |
| 32 x 107 | F, G | 32 (1.26) | 107 (4.21) |
| 38 x 250 | I | 38 (1.50) | 250 (9.84) |
| 38 x 258 | I | 38 (1.50) | 258 (10.16) |
| 38 x 314 | I | 38 (1.50) | 314 (12.36) |
| 38 x 370 | I | 38 (1.50) | 370 (14.56) |
| 54 x 250 | I | 54 (2.13) | 250 (9.84) |
| 54 x 326 | J | 54 (2.13) | 326 (12.83) |
| 54 x 403 | K | 54 (2.13) | 403 (15.87) |