How to measure antifreeze % in coolant in car?

I’d like to replace the coolant in my car myself but draining the rad doesn’t get all of it. I’d like to drain, refill with water, run the engine to circulate the water and repeat 5-6 times. Then drain the rad (now largely water) and backfill with enough antifreeze to bring it back to the recommended 1:1 ratio of water:antifreeze. I guess I could back calculate on how much antifreeze to add based on the stated volume of the system, but it would be more accurate if there was another way to analyze the coolant. Can it be done as for batteries? Checking the density?

By the way, any reason why I couldn’t briefly drive around with just water in the cooling system? To really allow a good exchange with the water flush with the old coolant?

It’s good you want to be thorough. If you’re letting the car fully warm up (fan cycling on/off), and you’re doing 5 (or even 2 or 3) drain/fill cycles, no need to drive around. It would be most accurate to find the specified volume of your cooling system, put half that much in anti freeze in there, then top off with water. You can get antifreeze hydrometers at the car parts place, but they’re approximate. Good to have to check with later, though, and they’re not big $$.

Just know that this isn’t critical, roughly 50/50 is what you want.

There’s several things you can do and yes, there is a gage similar to a battery hygrometer. You just suck up the solution and it will show the rating.

Sure, you can drive around with water in the system for a short time so long as there’s no freeze going on. But I will say that the antifreeze (ethylene glycol) aids much in lubricating and emulsifying the cooling system and fluid.

Most instruction manuals and antifreeze bottles will tell you however to replace with a 50/50% mix (water and antifreeze). That’s pretty simple and usually gives you protection to 25 to 35 below (zero).

There are anti-freeze testers for under $10 that do a good job. Based on their reading will tell you your ratio.

As for driving around with just water…Not needed. Once the car is warmed up it means the antifreeze is circulating and mixed up. I usually let it idle for 10-15 minutes. If you do drive…don’t do it for more then 10-15 minutes.

There is a tool that pros use to check coolant concentration and it is the only one I trusted as freeze protection was very important where I was doing most of my coolant work. It is is prizm (spygometer? not sure on spelling) type device. You put one drop of coolant on the lens and look through it and a mark appears on a scale telling you how cold you are safe for.

I never felt the floating ball type tools were accurate enough but then I had to put my signature on how good the coolant is and if I have to sign for something you can bet I will make sure what I sign is accurate.

While I support flushing the system with fresh water and then refilling, I’lll go further than teh others and suggest you NOT drive it around with only water. The coolant also acts as a lubricant for the water pump and there’s really absolutely no reason to wash that out and drive it around that way.

Honestly, the best approach is to drain the system, flush briefly with hose water, then refill with the proper 50/50 mix using distilled water (available at the grocery store for $1/gal). Some even consider the flushing with hosewater to be overkill. Be sure to burp the system if it has a burp valve at the high point.


I’ll bet what you used was a refractometer, something that measures the R.I. (refractive index). That used to be used in chemistry to measure solution compositions prior to access to gas chromatographs. Trouble is that it is prone to be off due to other impurities in the solution. I’m a chemist and I’ll see if I can’t borrow one for my work on the car. Should be simple to make up some standard solutions, measure the required RI and then match it.

I realize this is overkill since 50/50 isn’t critical, but I like the tech aspect to it.

If you really want to flush the system, perhaps you should install a flush-and-fill kit. With this you can hook up a garden hose and literally flush the whole system. Just make sure you drain the old coolant into an appropriate container first so you don’t pollute the ground water in your area.

not just a freeze - 50/50 mix not only freezes at a lower temp, it boils at a higher temp, lubricates the water pump itself and prevents rust.

Cool! We have a chemist in our midst!

Welcome. Stick around. We could often use your expertise.

Just in case you were not aware adding cold water to a hot engine while it is not running is a bad idea.

There are classier hygrometers than the floating ball type, but I found mine to be overkill and wasn’t broken hearted when something heavy fell on it. Since you are being meticulous, you might want to turn the heater on while draining. Maybe you will get some additional fluid out.

There are supposed to be drain bolts on the engine on some cars in addition to the drain at the base of the radiator, but they are often difficult to find. Opening them is supposed to get a full drain and also to somehow to solve the problem of air bubbles that can be difficult to coax out.

Going from memory (I took a degree in Chemistry many decades ago), 50-50 isn’t all that critical. If you actually end up with 45-55 instead, you’ll be fine.

It may be just me, but it seems to me that what used to be a simple, straightforward job has become more difficult in the past two decades. Maybe someone else has some insights.

The back of the bottle has a chart that tells you the level of protection of different proportions. No memory needed.

These are common tools available on all the tool trucks,you do have to calibrate the tool (very simple to do). It is a European way to test coolant freeze protection level.

This is really getting overcomplicated, the owners manual states the coolant capacity, if it is 6 gallons 3 gallons needs to be antifreeze un mixed. Sometimes after a complete water flush the system may not accept as much antifreeze as desired for a 50/50 mix, so be sure to drain everything as well as possible, including removing the lower radiator hose and letting that drain. If the amount of anttfreeze you add until system is full and is less than 1/2 capacity you have a problem and need the testing unit.

Some have reported that you can get quite a bit more fluid out of the block through the coolant (minus t-stat) outlet using a shop vac. I would think it might work blowing or sucking.

Good Idea, That is a new one to me but logical, thanks.

I just went thought this with my truck…all 28 qts! I got about 6 gal out. I have a cheapo Prestone tester. I have been searching for a good Hydrometer. Any suggestions for a DIY’er?

I found this:

Yup. Mix the coolant with distilled water per the ratios on the back of the coolant bottle and fill the system. It really doesn’t have to be turned into an exact science.

If you’re putting fresh coolant in every 3-4 years and mixing it per the bottle there’s realy no need to get that fancy.

I’d let the engine so the mixing. Drain, flush, drain, fill with recommended amount of antifreeze, top off with water.