How to Use a Drill Gage for Quick Screw and Bolt Sizing
By Products for bench top machinists | LittleMachineShop.com | Published: 2026-10-29
Category: How-to Guides
Learn how to use a drill gage to quickly identify screw, bolt, and drill bit sizes. A simple tool for bench top machinists.
Quick answer
A drill gage lets you identify screw, bolt, and drill bit sizes by inserting the fastener or bit into matching holes. The Drill Gage - BoltSize-It is a pocket-sized plastic card that resists oil and grease, making it ideal for shop use.
What a drill gage does and why you need one
A drill gage is a simple reference tool that helps you quickly identify the size of screws, bolts, and drill bits. Instead of guessing or measuring with calipers, you insert the item into the closest matching hole on the gage. This is especially useful when you have mixed fasteners in a bin or need to match a drill bit to a screw for a pilot hole.
The Drill Gage - BoltSize-It is made from sturdy, lightweight plastic that resists water, alcohol, oil, and grease. It fits in a shirt pocket (3-3/8" x 7-9/16") so you can keep it handy on the bench or in the field.
How to use a drill gage for quick sizing
- 1. Identify the fastener or bit you need to size: Pick up the screw, bolt, or drill bit you want to identify. Clean off any debris or grease so it fits into the gage holes smoothly. Common mistake: trying to force a dirty fastener into a hole. Clean it first for an accurate reading.
- 2. Insert the item into the closest matching hole on the gage: Hold the gage flat and insert the fastener or bit into the hole that looks closest in size. For screws and bolts, use the threaded portion; for drill bits, use the shank. The item should slide in without forcing. Checkpoint: if the item wobbles or is loose, try a smaller hole. If it won't enter, try a larger hole.
- 3. Read the size marking next to the hole: Once the item fits snugly, read the size printed next to the hole. For screws and bolts, this is typically the diameter and thread pitch. For drill bits, it's the bit size in inches or millimeters. Common mistake: reading the wrong marking if the gage has multiple scales. Double-check that you are using the correct side (e.g., screw vs. drill bit).
- 4. Verify with a second hole if needed: If you are unsure, try the next smaller or larger hole to confirm the fit. The correct size will feel snug but not tight. Checkpoint: if two holes fit equally well, the smaller one is usually correct for threaded fasteners.
- 5. Record or mark the size for later use: Once identified, write the size on a label or store the fastener in a sorted bin. This saves time the next time you need that size. Common mistake: assuming you will remember the size. Mark it immediately to avoid re-measuring.
Product facts
- Drill Gage - BoltSize-It: Sturdy and lightweight plastic resists water, alcohol, oil, and grease. Fits in shirt pocket (3-3/8" x 7-9/16" or 86 mm x 193 mm). You can keep it on you without worrying about shop fluids damaging it, and the compact size means it's always within reach.
Drill Gage - BoltSize-It
