I use an encounter calculator to try and balance my encounters for the party. They usually only do one or two a day, and they try to take long rests in between. I've never given them a deadly out of fear it would tpk, but hard encounters don't seem challenging. I've looked to critical role for inspiration, and in their first boss fights at level 8 with 8 people they were fighting elder brains, mind flayers, intellect devourers, and ogres in one encounter which is way above the deadly threshold. What should I do? 

Up the ante a little bit. It's not like you can't fudge some things behind the screen if things go south faster than you'd intended.

Encounters don't need to be winnable, and they also don't need to end in a TPK. Bargaining or fleeing are always options.

xguild's answer is quite on point here. I think the DMs here could give you some really specific advice if we knew what your group's character composition was, because quite frankly, CR is a only useful as a ballpark estimate of what a group can take on, and can sometimes be totally wrong. 

For instance, I had an encounter with four 2nd level PCs. Two clerics, a fighter, and a wizard. The group was able to defeat a CR 5 monster (a shambling mound, I think) without any deaths. Going off of CR alone, this would be considered "deadly," but group composition and player experience made all the difference. 

You can also stop caring if your encounters are balanced, and learn how to manage things going wrong in combat.

I typically don't try and balance encounters - save that I try not to make them too easy.

I may make them overly hard - but I also provide means for the characters to gather intelligence as to what they are facing, and they can decide if they are going to get into combat with those "critters" - or whether they'll try and circumvent them, or go get reinforcements, or ...

As I tell my players: I don't guarantee anything will be fair  - but there will always be a means for you to find out ahead of time, and make an informed choice ( which is a better deal than reality gives you ).

Also - as a DM, you can warp the combat subtly. Bad guys can make mistakes, or pull really well coordinated multi-pronged attacks, or run away, or knock out a player and take a hostage ( making the scene now a social/negotiation contest ), etc.

There are always ways out of an overpowered/under-powered combat scenario; learn those, and you needn't be quite so worried about precisely managing the CR balance.

The CR stuff provides a decent starting guideline, and you can generally manage encounters by sliding the creature's hit points one way or the other within their range.  For example, an Ogre has a hit point range of 28-91, with an average of 59.  I generally start an encounter with the average value, but will slide it one or another if the characters are proving to over- or under-matched as against my expectations.

What calculator do you generally use?

http://dhmstark.co.uk/rpgs/encounter-calculator-5th/

Much appreciated!

First, the CR calculator assumes multiple encounters per day.  The next chart in the CR section is called "Adventuring Day XP."  For a first level party, 100xp is a deadly encounter but the Adventuring Day is 3 (300 total XP).  

Now, my frustration with the WoTC published adventures is that they ignore the Adventuring Day and have too many times where the single planned encounter is double or triple times deadly.  But this does show that most times a party can "punch well above their weight" if they only have one encounter between long rests.

Quote from Windrunner >>

Now, my frustration with the WoTC published adventures is that they ignore the Adventuring Day and have too many times where the single planned encounter is double or triple times deadly.  But this does show that most times a party can "punch well above their weight" if they only have one encounter between long rests.