In RBY, a Pokemon's Critical Hit rate is determined by its base Speed; that is, the faster it is, the more likely it is to crit.  Surprisingly enough, this is the first resource to list the Critical Hit rate of every Pokemon in the game as well as its exact Speed stat (other guides only include the base Speed). 

Determining a Pokemon's Critical Hit rate: Simply multiply the Pokemon's base Speed by 100, and then divide by 512.  For example, Mewtwo's base Speed is 130, so 130 * 100 equals 13,000, and 13,000 divided by 512 equals 25.39, which means Mewtwo's Critical Hit rate is 25.39%.

The formula: Base Speed X 100/512

Critical Hit rates

Fully evolved Pokemon

1. Electrode - 27.34% (140 base Speed, 378 Speed)

2. Mewtwo/Aerodactyl/Jolteon - 25.39% (130 base Speed, 358 Speed)

3. Alakazam/Dugtrio - 23.44% (120 base Speed, 338 Speed)

4. Starmie/Persian - 22.46% (115 base Speed, 328 Speed)

5. Gengar/Tauros - 21.48% (110 base Speed, 318 Speed)

6. Rapidash/Scyther/Electabuzz - 20.50% (105 base Speed, 308 Speed)

7. Charizard/Mew/Dodrio/Fearow/Raichu/Zapdos/Tentacruel/Ninetales - 19.53% (100 base Speed, 298 Speed)

8. Raticate - 18.97% (97 base Speed, 292 Speed)

9. Arcanine/Primeape/Jynx - 18.55% (95 base Speed, 288 Speed)

10. Magmar - 18.16% (93 base Speed, 285 Speed)

11. Pidgeot - 17.77% (91 base Speed, 280 Speed)

12. Venomoth/Kangaskhan/Golbat/Mr. Mime/Moltres - 17.59% (90 base Speed, 278 Speed)

13. Articuno/Golduck/Nidoking/Hitmonlee/Pinsir/Seadra - 16.99% (87 base Speed, 268 Speed)

14. Gyarados - 15.82% (81 base Speed, 260 Speed)

15. Venusaur/Arbok/Kabutops/Dragonite - 15.63% (80 base Speed, 258 Speed)

16. Blastoise - 15.23% (78 base Speed, 256 Speed)

17. Hitmonchan/Nidoqueen - 14.84% (76 base Speed, 250 Speed)

18. Beedrill/Kingler - 14.65% (75 base Speed, 248 Speed)

19. Dewgong/Butterfree/Cloyster/Poliwrath/Onix/Victreebel/Magneton - 13.67% (70 base Speed, 238 Speed)

20. Seaking - 13.28% (68 base Speed, 234 Speed)

21. Hypno - 13.09% (67 base Speed, 232 Speed)

22. Sandslash/Flareon/Vaporeon - 12.70% (65 base Speed, 228 Speed)

23. Weezing/Lapras/Clefable/Tangela/Farfetch'd - 11.71% (60 base Speed, 218 Speed)

24. Exeggutor/Omastar/Machamp - 10.74% (55 base Speed, 208 Speed)

25. Chansey/Muk/Vileplume - 9.77% (50 base Speed, 198 Speed)

26. Ditto - 9.38% (48 base Speed, 194 Speed)

27. Golem/Wigglytuff/Marowak - 8.79% (45 base Speed, 188 Speed)

28. Rhydon/Porygon - 7.81% (40 base Speed, 178 Speed)

29. Snorlax/Slowbro/Lickitung/Parasect - 5.86% (30 base Speed, 158 Speed)

Not Fully Evolved Pokemon

1. Kadabra - 20.50% (105 base Speed, 308 Speed)

2. Haunter - 18.55% (95 base Speed, 288 Speed)

3. Poliwhirl - 17.59% (90 base Speed, 278 Speed)

4. Charmeleon - 15.63% (80 base Speed, 258 Speed)

5. Pidgeotto - 13.86% (71 base Speed, 240 Speed)

6. Dragonair - 13.67% (70 base Speed, 238 Speed)

7. Nidorino - 12.70% (65 base Speed, 228 Speed)

8. Ivysaur - 11.71% (60 base Speed, 218 Speed)

9.  Wartortle - 11.32% (58 base Speed, 214 Speed)

10. Nidorina - 10.93% (56 base Speed, 210 Speed)

11. Weepinbell - 10.74% (55 base Speed, 208 Speed)

12. Machoke - 8.79% (45 base Speed, 188 Speed)

13. Gloom - 7.81% (40 base Speed, 178 Speed)

14. Graveler/Kakuna - 6.83% (35 base Speed, 168 Speed)

15. Metapod - 5.86% (30 base Speed, 158 Speed)

Unevolved Pokemon

1. Voltorb - 19.53% (100 base Speed, 298 Speed)

2. Diglett - 18.55% (95 base Speed, 288 Speed)

3. Pikachu/Poliwag/Abra/Ponyta/Meowth - 17.59% (90 base Speed, 278 Speed)

4. Staryu - 16.99% (85 Speed, 268 Speed)

5. Gastly/Magikarp - 15.63% (80 base Speed, 258 Speed)

6. Doduo - 14.65% (75 base Speed, 248 Speed)

7. Rattata - 14.06% (72 base Speed, 242 Speed)

8. Tentacool/Mankey/Spearow - 13.67% (70 base Speed, 238 Speed)

9. Charmander/Vulpix - 12.70% (65 base Speed, 228 Speed)

10. Goldeen - 12.30% (63 base Speed, 224 Speed)

11. Horsea/Growlithe - 11.71% (60 base Speed, 218 Speed)

12. Pidgey - 10.93% (56 base Speed, 210 Speed)

13. Psyduck/Kabuto/Eevee/Ekans/Zubat - 10.74% (55 base Speed, 208 Speed)

14. Krabby/Weedle/Dratini/Nidoran M - 9.77% (50 base Speed, 198 Speed)

15. Seel/Caterpie/Venonat/Magnemite/Bulbasaur - 8.79% (45 base Speed, 188 Speed)

16. Squirtle - 8.39% (43 base Speed, 184 Speed)

17. Drowzee - 8.20% (42 base Speed, 182 Speed)

18. Nidoran F - 8.00% (41 base Speed, 180 Speed)

19. Shellder/Exeggcute/Sandshrew/Bellsprout - 7.81% (40 base Speed, 178 Speed)

20. Omanyte/Machop/Clefairy/Cubone/Koffing - 6.83% (35 base Speed, 168 Speed)

21. Oddish - 5.86% (30 base Speed, 158 Speed)

22. Paras/Rhyhorn/Grimer - 4.88% (25 base Speed, 148 Speed)

23. Jigglypuff/Geodude - 3.90% (20 base Speed, 138 Speed)

24. Slowpoke - 2.92% (15 base Speed, 128 Speed)

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