Let me start by saying: I've made every mistake you can make here. Picked a trowel too small, picked one too big-once I even used a drywall trowel (don't ask, I was in a hurry). But after years of fixing my own messes, I've got a system that works, and I'm gonna hand it to you-step by step, no guesswork.
First step: Check the tile's thickness. This is non-negotiable. If your tile is thin-like 1/4-inch thick (think small ceramics or mosaics)? You need a trowel with shallow notches, like 1/4-inch V or 1/4x3/8-inch square. Why? 'Cause thick mortar under a thin tile will make it bulge-you'll end up with a wavy wall or floor. I did a bathroom backsplash with 1/4-inch glass tiles once, used a 1/2-inch trowel, and the tiles looked like they were breathing. Had to scrape everything off and start over-total rookie move.
Second step: Look at the tile's size. Big tiles (12x24 and up) need more mortar to stay put-so you need deeper notches. A 12x24 porcelain tile that's 3/8-inch thick? Go for a 1/4x3/8-inch square notch. If it's 1/2-inch thick? Bump up to 1/2-inch square. Small tiles (6x6 or smaller)? Stick to 1/4-inch V or small square-you don't want mortar oozing between the grout lines. I laid a 16x16 natural stone floor once, used a small trowel 'cause I thought "size doesn't matter"-wrong. Half the tiles were loose within a week. Lesson learned.
Third step: Check the manufacturer's instructions. I know, I know-reading instructions is boring. But most tile boxes or mortar bags will tell you exactly what trowel to use. Last week, I was laying some fancy marble tiles, and the box said "use 3/8-inch square notch." I would've grabbed a 1/4-inch if I didn't read that-and I would've messed up big time. Save yourself the hassle, just flip the box over.
Last step: Do a test patch. This is my secret weapon. Spread a little mortar, stick a tile, press down gently. Then peel the tile up-you should see mortar covering 80-90% of the back. If there's big gaps? Your trowel's too small. If mortar's squishing out everywhere? Too big. Adjust, and test again. It takes 2 minutes, and it'll save you hours of fixing mistakes. Trust me, this step alone has turned my DIY jobs from "meh" to "professional-looking."
