Correctly measuring a block and tackle window balance is an important first step in finding a compatible replacement. You will need to identify the metal channel depth, measure the channel length, record the stamp, and compare the top and bottom guides.

Before You Start

Remove the balance from the window before measuring it. Measurements taken while the balance is still inside the jamb can be difficult to read accurately.

You may need:

• Tape measure
• Ruler or caliper
• Pen and paper
• Phone or camera for reference photos
• The original balance and plastic guides

Important: Measure the metal balance channel itself. Do not include the plastic top or bottom guides when recording the balance length.

Measure the depth of the metal channel from the open side, where the spring and cord assembly are located, to the flat back of the balance.

Common block and tackle balance channel depths include:

• 3/8 inch
• 5/8 inch

The balance shown in the image has a channel depth of approximately 5/8 inch.

Place the tape measure at one end of the metal channel and measure to the opposite end of the metal channel.

Do not include:

• The plastic top guide
• The plastic bottom guide
• Any projecting wings, tabs, hooks, or attachments

The example shown measures 15 inches from one end of the metal channel to the other.

Look along the metal channel for a printed, engraved, or stamped number. Common formats include:

• 1440
• 14-4

On these common stamp formats, the first portion of the stamp is typically one inch shorter than the physical metal channel length.

The remaining portion of the stamp helps identify the balance’s weight range or spring strength. Record the complete stamp exactly as it appears rather than relying on the channel length alone.

If the stamp is worn or unreadable, take a clear photo of the area and record the sash weight when possible.

The plastic attachments at the ends of the balance help connect and position it inside the window. These are commonly called the top guide and bottom guide.

Guide shape and dimensions can vary even when two metal channels appear similar. Before ordering, photograph each guide from multiple angles and compare:

• Overall guide shape
• Width and height
• Winged or non-winged design
• Hook, tab, or pulley position
• Mounting orientation
• Any molded numbers or markings

• Including the plastic guides in the channel-length measurement
• Measuring the balance while it is still installed
• Recording only part of the stamp
• Assuming the stamp is the physical channel length
• Ordering by appearance without comparing the guides
• Ignoring an unusual fractional channel length
• Discarding the original balance before confirming the replacement

These related resources can help with the remaining identification and replacement steps:

• Understanding Block and Tackle Balance Stamps
• Selecting Top and Bottom Guides
• Replacing Non-Tilt Block and Tackle Balances

To measure a block and tackle balance, record the channel depth, measure only the metal channel, copy the complete stamp, and compare the plastic top and bottom guides.

Accurate measurements and clear photos make it much easier to narrow down a compatible replacement.