How to Size a Disc Harrow Blade for Your Machine

John Deere tractor pulling a red disc harrow through Iowa cropland

Picking the right replacement disc blade comes down to four measurements — nail those and everything else falls into place. This guide walks you through each in the order you should confirm them.

1. Diameter

Measure the outside diameter of your worn blade in inches. Disc harrow blades come in a specific set of sizes: 14", 16", 18", 20", 22", 24", 26", 28", 30", 32", 36", and larger. If your blade is worn down enough to be smaller than the original, look up the original diameter in your machine's manual or match to the mount hole spacing.

Common pairings by machine class:

  • Light-duty disc harrows (residential, small garden): 16"-20"
  • Standard farm disc harrows: 20"-24"
  • Heavy-duty offset discs: 24"-28"
  • Vertical tillage / high-speed disks: 20"-24" (thinner, faster rotation)

2. Thickness

The second-most important spec after diameter. Thickness is measured in millimeters (metric) or fractions of an inch. Common thicknesses:

Metric Fraction Typical use
4.5 mm 3/16" Light-duty, small acres
6 mm 1/4" Standard field cultivator / disc harrow
6.5 mm — Standard heavy tillage
8 mm 5/16" Heavy offset / rocky ground
10 mm 3/8" Extreme heavy-duty, dry-hard soil

Thicker blades last longer but add weight to the machine and demand more tractor HP. If your soil is soft and you're not fighting rocks, don't over-buy.

3. Axle bore

The hole in the center of the blade needs to match the axle it slides onto. Two shapes exist:

  • Square axle — most disc harrows. Common sizes: 1", 1-1/8", 1-1/4", 1-1/2", 2".
  • Round axle — some older or specialty rigs. Common: 1-3/4", 2".

Measure diagonally corner-to-corner for square axles. If the old blade is on a spool with bushings, verify the actual axle size, not the bushing OD.

4. Edge (notched or smooth)

Choose based on residue and soil conditions — see our Notched vs. Smooth guide for the full breakdown.

Quick order checklist

  1. Diameter (inches)
  2. Thickness (mm or fraction)
  3. Axle bore (size and shape)
  4. Edge (notched or smooth)
  5. Number of blades needed

If you can send us those five specs, we can quote you the same day. Call 641-425-8034 or email sales@discbladesforsale.com.

Frequently Asked Questions

How do I measure a disc harrow blade for replacement?
Pull one off your gang and measure across the full face of a fresh (unworn) blade if you have one available; otherwise measure a worn blade and round up to the next standard size (16, 18, 20, 22, 24, 26, 28"). Also measure the center-hole size and shape (round or square) — those two numbers plus the diameter are what you need to buy right.
What size disc blade do I need for my harrow?
Match the diameter, thickness, and axle-hole size on the blade currently installed. Most US disc harrows use 20", 22", or 24" blades with a 1-1/8" or 1-1/2" square axle. If you're unsure, text a photo of your old blade to 641-332-4102 and we'll cross-check the specs before you order.
Do disc blades wear down in diameter?
Yes — blades wear from the outside edge in. A blade that started at 24" can end its service life around 19-20". Never buy a blade at the current worn diameter; buy at the original manufactured diameter or you'll be a full size undersized after one season.
What thickness disc blade should I buy?
Match the original thickness stamped on your blade. Common thicknesses are 3/16" (light-duty finish work), 1/4" (general tillage), and 5/16" or 3/8" (primary tillage / rocky soil). Going thicker adds weight and holds edge longer; going thinner cuts cheaper but wears faster.
What's the difference between a round and square axle hole?
Most modern disc harrows use a square axle (typically 1-1/8", 1-1/4", or 1-1/2"). Older machines and some European brands (Lemken, Kverneland) use round axles. The wrong shape won't fit — this is the #1 return reason on replacement blades. Confirm the axle shape before ordering.