How to Measure Lineset Length for Your Mini Split

So you're thinking about getting a mini split HVAC system but want to make sure you order the correct line sizes for your system? Smart move.  

The good news? Measuring for lineset length isn't rocket science. You just need a tape measure, a little patience, and the ability to resist the urge to eyeball everything. Let me walk you through it.

Why Lineset Length Actually Matters

Before we dive into the how-to, let's talk about why you can't just wing this.

Every mini split has a minimum and maximum lineset length, usually something like 15 feet minimum and 50 feet maximum. Go under the minimum, and you'll run into issues that'll make your compressor very unhappy. Go over the maximum, and the system won't cool efficiently (or at all). MRCOOL for example has a lineset maximum length of 75ft, by connecting 2 lines together using a coupler (50ft connected to a 25ft).

Also, if you accidentally get the wrong size, you'll have to deal with returning it and waiting for the new one to get there. This can cost valuable time and money. 

What You'll Need

Keep it simple:

• Tape measure or measuring wheel: A 50-foot tape measure works great for most residential installs

• Sketch or floor plan: Even a napkin drawing helps you visualize the route

• Ladder: If you're measuring vertical sections or routing through an attic

• Marker or chalk: For marking key points along the way

Quick Tip: Take photos along your intended route with your phone. It helps when you're ordering materials or explaining the install to someone else, and saves you from crawling back into that dusty attic for a second look.

The Step-by-Step Process

1. Find Your Connection Points

Start by identifying where the line set will actually connect:

• Indoor unit port: This is where the refrigerant lines hook up to your indoor evaporator coil on the air handler. The lines can go straight out the back or to the left or right.

• Outdoor unit port: Where the lines connect to your outdoor condenser unit

Mark these spots. They're your start and finish lines.

Pro Trick: Use painter's tape to mark your intended path along walls and ceilings before you measure. This gives you a visual guide to follow and helps you spot potential problems, like that electrical box you forgot was there.

2. Trace the Actual Route

Here's where people mess up: they measure the straight-line distance between the units and call it a day. Don't do that.

Instead, follow the actual path the lineset will take. This includes:

• Horizontal runs along walls, ceilings, or through crawl spaces

• Vertical sections going up or down between floors, into attics, or down exterior walls

• Bends and turns around corners, through studs, or around obstacles

Pro tip: Measure around corners and obstacles. Don't cut diagonally through your mental wall. If the lineset has to go around a door frame or navigate a tight corner, measure that extra distance. Those bends add up.

Think of it like giving directions to someone who's never been to your house. You wouldn't say "just head 30 feet northeast." You'd say "go down the hallway, turn left at the bathroom, then straight to the back door." Same logic here.

Insider Trick: Run a piece of string along your intended path first. This helps you visualize bends and obstacles you might have missed. Once you've got the string in place, measure it. This is especially helpful for complex routes with multiple turns.

3. Add Extra for Fittings and Slack

Don't cut it close. Literally.

Add an extra 1 to 2 feet on each end to allow for:

• Proper flaring and connection at both units

• Routing the line into equipment ports without putting stress on the connections

• A little breathing room in case you need to adjust anything during installation

If your route has multiple tight bends or tricky clearances, toss in a bit more slack. The future you will thank the present you.

Pro Tip: Linesets typically come in 5-foot increments (15 ft, 25 ft, 35 ft, 50 ft). If your measurement comes out to 32 feet, order the 35-foot lineset. It's better to have a little extra than to come up short and have to splice lines or start over.

4. Do the Math

Now add it all up:

• All horizontal segments

• All vertical segments

• Your allowances at both ends

That total? That's your lineset length.

Sanity Check: Write down each segment as you measure it. "10 ft living room wall, 8 ft vertical exterior, 15 ft ground run to condenser, 2 ft slack = 35 ft total." Your brain will forget the numbers by the time you walk back inside, especially if you're juggling a ladder and a tape measure.

Real-World Example

Let's say you're installing a mini split in a bedroom with the condenser outside:

• 10 feet horizontal inside the room to the exterior wall

• 8 feet vertical drop down the outside wall to ground level

• 15 feet horizontal run along the ground to the condenser pad

• 2 feet extra for slack and bends

Total lineset length = 10 + 8 + 15 + 2 = 35 feet

Easy, right? Now you know you need a 35-foot lineset.

Common Mistakes to Avoid

Measuring "as the crow flies": The shortest distance between two points doesn't account for walls, floors, or the laws of physics. Measure the real route.

Forgetting vertical sections: That 10-foot drop from a second-floor bedroom to the ground? Yeah, that counts.

Not adding slack: Tight connections strain fittings and make installation a nightmare. Give yourself room to work.

Ignoring manufacturer specs: Check your mini split's manual for minimum and maximum lineset lengths. Ignoring these isn't just bad practice. It'll void your warranty.

Assuming all routes are equal: The shortest route isn't always the best route. Sometimes going a bit longer to avoid obstacles or maintain better access is worth the extra foot or two of lineset.

Final Thoughts

Measuring lineset length for a mini split isn't complicated, but it does require some attention to detail. Take your time, follow the actual installation path, and add a little extra for good measure. Your system will run better, last longer, and you'll avoid the headache of having to redo everything because you came up short.

To explore line sets for MRCOOL systems go here. Also if you want to talk to someone about your mini split install, just give us a call or message us in our chat.

Good luck!