how to figure out how strong someone is

I have a problem. Lately I have been playing lots of games against my grandfather. All of them are played on a board and I don't know how to figure out what his real rating is. He never competed in rated tournaments and neither have I. He is better than me and I'm curious to find out our rating difference. I know my OTB rating compared to this one would be lower. We rarely have draws and 95% of our games are decisive. If I had to guess I would say that I win one out of three or four games. Its hard to tell because for example yesterday I started out with 2 wins, 2 draws and only later he managed to beat me overall. I have a good start but fail to keep it that way. Every night he is a winner overall, but I still keep games fun and interesting to play for both of us. By that I mean I always try my best to not be easy to beat and its working out sometimes. My question is - is there a way I can find out what is his rating, is there a site for that or something like that? What would be your guess if I asked you what rating is my grandfather at based on what I have said so far. When he gets serious about a game, like really focused, concentrated and thinks about a move for solid amount of time, I don't think I would win even 1 out of 10 games(lets say any time format longer than 30 minutes which we don't play). We play without clocks and one game is finished by five to ten minutes tops. If I missed any important info let me know. Thank you for reading and have a nice day! :)

Well winning 1 out of 4 translates to a 200 point rating difference, so if one of you gets rated, you can make an estimate of the other based on that.

There are charts to see rating difference vs expected win percent.

But of course style can affect 2 people also. Like if you feel psychologically lost against a particular opponent for some reason, like grandpa just a always beats me!

Bobby Fischer always lost to some guy named Geller.

Mikhail Tal always lost the first game in a tournament.

Interesting, but I don't think I have that problem because I'm not thinking before the game that I already lost. Im also convinced that in couple of months we will have same strength and in one year I will be able to surpass him in chess. Thank you for your answer.

Both of u need to go play a bunch of tournaments together. It'll make chess more fun than either of u ever expected. Talk him into it. Nobody likes the woulda, coulda, shoulda issue. Just go do it, go for it, where's ur sense of adventure? 

I don’t have an answer for your question. I just want to say how blessed you both are to be able to spend time with one another. I would cherish the same opportunity with my ancestors. Well played my friend.

Yes. My grandpas were dead before I was 8, but dad taught me chess and we played many games together.

I understand, mine were both gone by age five. I’ve wondered my whole life what might have been. 

There’s a thing called ratings

     You say you play 5-10 minute games and he wins about 66% of the games vs 29% for you and 5% draws, so he's probably a good 150 elo points better at blitz. But you also feel that in a longer game he would win almost all of the games (not necessarily true as you might find stronger moves if you took the time to look for them) so NO, there is no way to figure out either of your real strength. 

     Go play a couple of OTB tournaments, get a rating, play some longer games with him, and you'll be able to make a better guess.

They really do

Well, I can defeat Sven bot (1100) and lost 80% of times to Nelson bot (1300) so I am about 1200.

Ask to your father play against chess.com bots, and see what bots he can defeat.

#5 I will deinitely play tournaments sometimes in future, but not right now. My grandfather is not interested in that, he is old and he thinks it's not his thing.
#6 Thank you for your comment anyways. It is really a blessing to be able to spend time with your friends and family, especially elders. I have been lucky to never experience death of a close one until last year when my other grandfather died. At that moment I realized how important it is to be around people we love as much as possible.
#14 bots are overrated and they would give me same rating as I think he has

 It's true.

I did read it also.

There are comfortable and uncomfortable opponents for everyone.

It depends a lot on personal Chess style.

Possibly  the way Efim Geller played Chess was hellish for Bobby Fisher... and the great champion didn't know how to beat his opponent.

There are elo calculators for tournaments. You could use your rating as opponent and your game results as score to estimate his rating. Of course it is not very precise if he just plays against you...   

I'd estimate him to be around 1400? Beating an 1800 1/5 times (If I'm reading that right) sounds around that skill level to me

#17 thank you for your suggestion, I will try it.

#18 1400 OTB? What would be his chess.com rating then, 1900? I (1800 online) beat him 1/3 - 1/5 times depends on the day. Neither of us has FIDE or USCF rating because we never played any tournaments, rated or not. 

well, using how elo expected ratings are calculated,

1/3 win percent for you would be 121 above yours 

1/5 win percent for you would be 240 above yours

So he can be anywhere between 1900-2100 online, thabk you for answering me that question. Do you know how to convert this to OTB rating?  Also could you show me how or where you got those numbers, Im interested to see how that works

1400 rapid on cc is what I was thinking