The Quick Answer

 A hydrometer is one of the most important tools in home brewing. It measures the gravity of your wort and beer, allowing you to track fermentation, confirm when it's finished, calculate alcohol content and troubleshoot brewing problems. If you're not taking gravity readings, you're basically brewing blind.

The Most Underrated Tool in Home Brewing

If there's one piece of brewing gear that doesn't get enough love, it's the humble hydrometer.

Everyone gets excited about shiny fermenters, pressure kits, glycol chillers and fancy brewing systems. Fair enough too — new gear is fun.

But if you asked me which piece of equipment gives you the most useful information about your beer for the least amount of money, I'd point straight at a hydrometer.

For the price of a couple of schooners, it tells you exactly what's happening inside your fermenter.

And unlike your mate who reckons he can tell if a beer is finished "just by looking at it", a hydrometer is actually right.

Why You Can't Judge Fermentation by Appearance

I was reminded of this recently while brewing our Cream Stout Fresh Wort Kit.

The fermentation looked... well... a bit underwhelming.

After about 10 to 12 days there wasn't much krausen or many other signs that anything exciting had happened.

Looking at the fermenter alone, I'd convinced myself something had gone wrong.

Maybe fermentation had stalled.

Maybe the yeast had packed it in.

Maybe it wasn't finished.

But appearances can be deceiving.

That's why we use a hydrometer.

What My Hydrometer Told Me

The Cream Stout Fresh Wort Kit starts with an Original Gravity (OG) of around 1.070.

After topping up with roughly five litres of water, my starting gravity came in at about 1.056.

When fermentation was complete, I took another reading.

Final Gravity (FG): 1.014.

Straight away, I knew:

• Fermentation was complete

• The yeast had done its job

• The beer had finished at 1.014 FG, which allowed me to calculate an ABV of around 5.5%

• It was ready for conditioning and packaging

The beer looked ordinary.

The numbers looked perfect.

That's the difference a hydrometer makes.

How to Calculate the Alcohol Content of Your Beer

One of the handiest things a hydrometer does is help you work out the alcohol content of your beer.

Once you've got your Original Gravity (OG) and Final Gravity (FG), calculating ABV is pretty straightforward.

Use this formula:

$ABV=(OG-FG)\times131.25$

Using my Cream Stout as an example:

• Original Gravity (OG): 1.056
• Final Gravity (FG): 1.014

The calculation looks like this:

(1.056 - 1.014) × 131.25 = 5.5% ABV

That's how I knew the beer had landed right where I wanted it.

Now, don't stress if maths isn't your thing. There are plenty of online ABV calculators around, and most brewing software will do the calculation for you.

But understanding where the number comes from helps you better understand what's happening in your beer.

So, What Does a Hydrometer Actually Measure?

A hydrometer measures the density of liquid compared to water.

Before fermentation, your wort contains lots of dissolved sugars which make it heavier than water.

As yeast consumes those sugars and converts them into alcohol and carbon dioxide, the density drops.

That drop in density is what your hydrometer measures.

By comparing your Original Gravity (OG) and Final Gravity (FG), you can:

• Track fermentation progress

• Confirm fermentation is finished

• Calculate alcohol content

• Spot potential fermentation issues

• Keep accurate brewing records

• Better understand how your beer is performing
  

How to Use a Hydrometer

Using a hydrometer is simple.

Step 1: Take an Original Gravity Reading

Before pitching your yeast, collect a sample of wort and take a gravity reading.

Record this number.

This is your Original Gravity (OG).

Step 2: Let Fermentation Do Its Thing

Leave the fermenter alone and let the yeast get to work.

Depending on your beer style and fermentation temperature, this could take anywhere from a few days to a couple of weeks.

Step 3: Take a Final Gravity Reading

Once fermentation appears to have slowed, take another gravity reading.

Step 4: Check Again in 48 Hours

Take a second reading a day or two later.

If the gravity hasn't changed, fermentation is complete.

If it's still dropping, leave it a little longer.

How Do You Know When Fermentation Is Finished?

This is one of the most common questions we get in the shop.

"Stu, do you reckon my beer's finished?"

The honest answer is always the same.

"I don't know. What's the gravity reading?"

Because:

• Airlocks can stop bubbling while fermentation continues

• Krausen doesn't always form during fermentation

• Different yeast strains behave differently

A stable gravity reading is the only reliable way to know fermentation is finished.

Keep Records and Become a Better Brewer

One thing experienced brewers do really well is keep records.

Every batch teaches you something.

Recording things like:

• Original Gravity (OG)

• Final Gravity (FG)

• Fermentation temperature

• Yeast strain

• ABV

• Brewing dates

• Tasting notes

makes it much easier to repeat great beers and troubleshoot the occasional disaster.

The more notes you keep, the more consistent your brewing becomes.

Don't Brew Blind

Your beer might look finished.

It might look stalled.

It might look like absolutely nothing is happening.

But the fermenter can be a terrible liar.

A hydrometer tells you what's actually going on inside your beer.

It's one of the cheapest pieces of equipment you'll ever buy and one of the most valuable.

If you're not already using one, grab a hydrometer and start taking gravity readings.

Your future self — and your beer — will thank you.

Frequently Asked Questions

What gravity should my beer finish at?

Every recipe is different, but most beers finish somewhere between 1.008 and 1.020. Check the recipe or kit instructions for guidance.

Can I bottle beer without using a hydrometer?

You can, but it's risky. Bottling before fermentation is complete can create over-carbonated bottles and potentially dangerous bottle bombs.

Is a hydrometer better than watching the airlock?

Absolutely. Airlock activity only tells you gas is escaping. A hydrometer tells you what the beer is actually doing.

How accurate are hydrometers?

When used correctly and temperature corrected if necessary, hydrometers are very accurate and remain the industry standard for measuring gravity.

What's the difference between a hydrometer and a refractometer?

A hydrometer measures liquid density directly. A refractometer uses light refraction. Both work well, but hydrometers are generally simpler and more affordable for beginner home brewers.