Dog / Human Equivalent, in Years

Your Dog's Actual Age	Small dog	Medium Dog	Large Dog	Giant Breed Dog
3 mo	9	8	6	5
6 mo	13	12	11	9
1 yr	21	18	16	14
2 yr	28	25	22	21
3 yr	31	28	25	30
4 yr	33	30	28	35
5 yr	35	36	38	40
6 yr	38	40	42	45
7 yr	40	45	50	55
8 yr	43	50	55	60
9 yr	46	55	60	65
10 yr	50	60	65	70
11 yr	55	65	70	75
12 yr	60	70	75	85
13 yr	65	75	80	90
14 yr	70	80	85	95
15 yr	75	85	90	100
16 yr	80	90	95
17 yr	85	95	100
18 yr	90	100
19 yr	95
20 yr	100

Smaller dogs tend to mature rapidly and age slowly.  Larger dogs take longer to mature and age very quickly.  These ages are only approximate.  Some breeds and individuals will vary from this chart.
 

Cat

Your Cat's Actual Age	Human Equivalent, in Years		Your Cat's Actual Age	Human Equivalent, in Years
3 mo	6		11 yr	60
6 mo	12		12 yr	65
1 yr	18		13 yr	68
2 yr	24		14 yr	70
3 yr	28		15 yr	75
4 yr	30		16 yr	78
5 yr	34		17 yr	80
6 yr	38		18 yr	85
7 yr	45		19 yr	90
8 yr	50		20 yr	95
9 yr	55		21 yr	100
10 yr	58

Cats mature much more rapidly than humans.  Cats reach maturity in less than 10% of their life span; humans take 25% of their life span to mature.