Metric vs Imperial: Complete Conversion Reference Table
Reviewed by Jerry Croteau, Founder & Editor
Table of Contents
I was standing in the lumber aisle doing math on my phone and nothing was adding up
I had a cart with a couple 2x4s, a tape measure, and that weird confidence you get right before you realize you’re mixing units.
The plan was simple: grab a few metric fasteners, check a cut list somebody texted me in millimeters, and sanity-check it against a tape that’s in feet and inches. And then my brain did that thing where it pretends it remembers conversions and it absolutely does not.
So I opened my notes and wrote down the handful of conversions I actually use in real life, and honestly, once you’ve got the “anchor” numbers, the rest stops being scary.
The conversions you’ll use constantly (and the ones you’ll pretend you don’t)
You don’t need a wall of math. You need a few reliable pivots you can trust when you’re splitting a bill, ordering materials, reading a recipe, or converting a spec sheet that somebody sent from the other side of the planet.
Here are the anchors I keep coming back to:
millimeters = inches × 25.4
feet = meters ÷ 0.3048
meters = feet × 0.3048
F = (C × 9/5) + 32
C = (F − 32) × 5/9
And yeah, I had no idea why 25.4 was the magic number at first. I nodded like I understood. I didn’t. Then I finally accepted it’s just a defined relationship: 1 inch is exactly 25.4 mm. So you stop arguing with it and you use it.
So why does everyone get this wrong?
Because we mix “exact” conversions (like inches to mm) with “practical” rounding (like 3 feet is about 0.91 meters) and then we act surprised when the total comes out weird.
Complete conversion reference table (print it, screenshot it, whatever)
This is the part you came for. I’m putting the common stuff in one place so you don’t have to bounce between tabs while you’re trying to get something done.
| Category | Imperial → Metric | Metric → Imperial | Quick mental note |
|---|---|---|---|
| Length (small) | 1 in = 25.4 mm | 1 mm ≈ 0.03937 in | 25 mm is basically 1 inch |
| Length (medium) | 1 ft = 0.3048 m | 1 m ≈ 3.28084 ft | 1 m is a bit over 3 ft |
| Length (long) | 1 mi ≈ 1.60934 km | 1 km ≈ 0.62137 mi | 5 km is about 3.1 miles |
| Weight / mass | 1 lb ≈ 0.453592 kg | 1 kg ≈ 2.20462 lb | 2.2 lb per kg |
| Volume (kitchen-ish) | 1 fl oz ≈ 29.5735 mL | 1 mL ≈ 0.033814 fl oz | 30 mL is about 1 fl oz |
| Volume (bigger) | 1 US gal ≈ 3.78541 L | 1 L ≈ 0.264172 US gal | 4 L is roughly a gallon |
| Area | 1 sq ft ≈ 0.092903 m² | 1 m² ≈ 10.7639 sq ft | 10 m² ≈ 108 sq ft |
| Temperature | F → C: (F − 32) × 5/9 | C → F: (C × 9/5) + 32 | 0 C = 32 F, 100 C = 212 F |
One thing I’ll say: the “quick mental note” column is where the real value is. That’s the stuff you can do while you’re holding a tape measure in one hand and your phone in the other.
And if you’re doing anything even mildly expensive, don’t wing it. Get the exact number.
A worked example (because tables are nice, but you’re trying to finish something)
Say you’re splitting a bill with friends and the receipt is in liters because you’re traveling, but you think in gallons. You bought 18 liters of fuel (or whatever liquid) and you want to know roughly how many gallons that is, and then you’re dividing it three ways.
Step 1: Convert liters to gallons using the anchor: 1 L ≈ 0.264172 US gal.
18 L × 0.264172 ≈ 4.755 gal (call it about 4.76 gallons).
Step 2: Split it three ways.
4.755 ÷ 3 ≈ 1.585 gal each.
So if your buddy says “I only used like 1 gallon,” you can politely point at the math and be like… yeah, no. It’s closer to 1.6.
But here’s the sneaky part: if you instead use the mental shortcut “4 liters is roughly 1 gallon,” then 18 liters becomes about 4.5 gallons, and that’s not insane, but it’s not the same. On a road trip, you probably don’t care. On a job where you’re ordering material, you might care a lot. That’s the line.
So yeah, rounding is fine. Rounding without admitting you’re rounding is where the excessiveness starts.
Use the calculators (I built these because I got tired of re-checking myself)
I’m biased because I run ProCalc.ai, but I made these because I kept doing the same conversions over and over and over, and I wanted something that didn’t feel like a science fair project.
If you just need a clean conversion fast:
I’ll be honest: I still keep the anchor numbers in my head, because it’s faster than typing, but the calculator is how I avoid those “wait… did I multiply when I should’ve divided?” moments.
And those moments happen to everyone. Everyone.
FAQ (the stuff people ask right after they say they’re “bad at math”)
Is 25 mm really close enough to 1 inch?
For quick estimating, yeah. 1 inch is 25.4 mm exactly, so 25 mm is short by 0.4 mm. If you’re laying out holes across a long run, that tiny error can stack up, so switch to exact when precision matters.
Why does converting temperature feel harder than converting length?
Because it’s not just a multiplier. You’re scaling and shifting. The “+32” part is the shift, and the “× 9/5” part is the scale. Once you accept it’s a two-step thing, it stops being mysterious.
What’s the fastest way to do meters to feet in your head?
I do it like this:
- Start with “1 m is about 3.28 ft.”
- Multiply by 3, then add a bit more (about a quarter of the meters) to cover the 0.28 part.
- If I’m ordering materials or cutting anything, I don’t trust my head and I use the calculator.
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