The French words tant and autant are both adverbs of quantity, but their meanings and uses are different. Autant means as much/many and is usually used in comparisons. Tant means so much/many and tends to be used to intensify. Take a look at the following summary table for more details.

	TANT - So much, many		AUTANT - As much, many
Tant and autant (que) modify verbs.
	Il a déjà tant fait.		Faites autant que vous pouvez.
	- He has already done so much.		- Do as much as you can.
	Il travaille tant !		Je travaille toujours autant.
	- He works so much!		- I work as much as always.
Tant de and autant de modify nouns.
	Il a tant d'amis.		Il a autant d'amis que toi.
	- He has so many friends.		- He has as many friends as you.
	Ta maison a tant d'espace !		Ma maison a autant d'espace.
	- Your house has so much space!		- My house has as much space (as this one).
Tant (que) intensifies, while autant que equalizes.
	Il a tant mangé qu'il est malade.		Il a mangé autant que toi.
	- He ate so much that he's sick.		- He ate as much as you.
	J'ai tant lu que j'ai mal aux yeux.		C'est pour lui autant que pour toi.
	- I read so much that my eyes hurt.		- It's for him as much as for you.
	J'ai dû partir, tant j'étais fatigué.		Il est sympathique autant qu'intelligent.
	- I had to leave since I was so tired.		- He is as nice as he is intelligent.
Tant que can also mean while, as long as, or since.
	Tant que tu habiteras ici, tu m'obéiras.
	- As long as you live here, you will obey me.
	Tant que tu es là, cherche mes lunettes.
	- As long as / since you're here, look for my glasses.
Tant can replace autant in a negative or interrogative sentence.
	Je n'ai pas mangé tant que toi.		J'ai mangé autant que toi.
	- I didn't eat as much as you.		- I ate as much as you.
	A-t-il tant d'amis que toi ?		Il a autant d'amis que toi.
	- Does he have as many friends as you?		- He has as many friends as you.
Tant can express an indefinite quantity.
	Il fait tant par jour...
	- He does so much (x amount) per day...
	tant pour cent
	- such and such a percent
Expressions
	en tant que	as, like	autant... autant	as... as
	tant bien que mal	as best one can	autant que possible	as much as possible
	tant et plus	a lot	c'est autant de	that's ... at least
	tant et si bien que	so much so that	comme autant de	like so many
	tant il est vrai que	since, as	d'autant	accordingly, in proportion
	tant mieux	so much the better	d'autant mieux	even / all the better
	tant pis	never mind, too bad	d'autant moins	even less
	tant qu'à	may as well	D'autant plus !	All the more reason!
	tant s'en faut	far from it	d'autant plus... que	all the more so
	tant soit peu	remotely, at all	en... autant	the same
			pour autant	for all that
			pour autant que je sache	as far as I know