3D Printer Belt Tensioning: How to Check and Adjust Your X/Y Belts

What a badly tensioned belt does to your prints

On an FDM printer, toothed belts carry the motion of the stepper motors to the print head and the bed. The Prusa Knowledge Base lists layer shifting, ghosting and other print abnormalities as consequences of a loose belt, including a cylinder that prints with an irregular shape instead of a perfect circle. With slack in the system, the head overshoots every time it changes direction, then catches up. Your walls record those little oscillations.

Going too far the other way doesn't help. Prusa's CORE One article notes that an overtightened belt can cause irregular movement in the X and Y axes. So the goal is not to crank the belt as tight as it will go. You want enough tension, applied consistently and holding steady over time. If you're seeing echoes around letters and sharp corners, our guide to ghosting and ringing helps you figure out whether the belts or your print speed are to blame.

Finding your X and Y belts on bed slingers and CoreXY

On a bed slinger such as the Prusa MK4 or MK3, the Y-axis belt runs under the heatbed and the X-axis belt moves the print head, as Prusa's documentation explains. Each belt has its own tensioner and is adjusted on its own. On a CoreXY machine like the CORE One, the two belts work together to move the head across the plane, so you'll see them described as the upper belt and the lower belt rather than X and Y.

That difference changes how you work. On CoreXY, a tension mismatch between the two belts can rack the gantry so it's no longer square. Prusa tells you to check gantry alignment before touching tension, with the motors disabled. If you're not sure which motion system your printer uses, our CoreXY vs bed slinger explainer covers both. For enclosed printers from other brands, check the manufacturer's documentation, since tensioner access varies a lot from one model to the next.

Checking tension by sound and feel

A manual check by sound and feel is still the starting point. Per Prusa, a well-tensioned belt sounds roughly like a low bass string when you pluck it. You should be able to pinch the two sides together between your thumb and index finger, but feel a little resistance. A belt that folds with no effort is too loose. One that won't give at all is probably too tight.

A second check reveals mechanical slack. Prusa's assembly manual suggests holding the X motor shaft with pliers, using the flat part of the shaft, then pushing the extruder by hand. You shouldn't see any slack appear in the belt. The same procedure works for the Y axis. If slack shows up even though the belt feels tight, the pulley may not be properly tightened on the shaft. Run this check in the same spot every time, with the head centered on the axis, so you can compare results over time.

Prusa's belt measurement tools

On the MK3/S/+, the LCD menu under Support and then Belt Status shows a number for each axis. Prusa is clear that this isn't a physical quantity but an arbitrary number inversely proportional to motor load. The suggested ballpark is around 250 on X and 275 on Y, with an optimal range of plus or minus 15. If you're at or near 240, loosen the belt. At or near 290, tighten it. Those numbers assume a good assembly with parts in good shape and properly lubricated. They refresh every time you run the Selftest or the Belt test under Calibration.

For newer machines, Prusa offers a belt tuner on its website and in the Prusa App. It uses your phone's microphone to measure the vibration frequency when you strum the belt. On the CORE One, starting with firmware 6.4.0, a built-in wizard under Settings and Manual Belt Tuning excites the belt and uses stroboscopic feedback to show its resonance. Prusa lists a maximum of 98 Hz for the CORE One's upper belt, and the screen then tells you whether to tighten or loosen.

Adjusting belt tension step by step

Every adjustment on Prusa's MK printers uses the 2.5 mm hex key. For the X axis on the MK4/S, MK3.9/S, MK3.5/S and MK3S+, slightly loosen the screws holding the motor first, or the tensioner can't move it. Then tighten the screw on the back of the X-end-motor with the ball end of the key, checking tension every turn or two, and snug the motor screws back down. For the Y axis, the idler sits behind the LCD: two screws pass through the front plate, and turning them clockwise closes the gap between the idler and the frame.

If fine-tuning isn't enough, Prusa describes a bigger adjustment at the belt holder. Mark the belt with a pen right next to the plastic part so you know where it was, then move it one or two teeth. On CoreXY, always alternate between the two tensioner bolts and turn each one by the same amount to keep the gantry square. On CORE One kits, Prusa also reminds you to lubricate the tips of the tensioner screws. Once you're done, measure again and run a quick print to confirm.

What belt tension won't fix

Not every layer shift comes from a loose belt. A pulley slipping on the motor shaft, a nozzle catching on a curled-up part or simply too much speed can look very similar. Our layer shifting guide covers those other causes. Likewise, if some ghosting remains once tension is right, input shaping in the firmware can often reduce it, provided your printer supports it.

Also, don't mix up belt tension with extruder tension. The Prusa Knowledge Base has a separate article on idler screws, which press the filament against the drive gear. Too loose and the gear can't grab the filament. Too tight and the extruder motor works too hard and can start grinding it. That's an extrusion setting, and it has no effect on ghosting. Finally, make belt checks part of your regular 3D printer maintenance routine so you catch a belt going slack before it shows up in your parts.

Sources and limits

• Prusa Knowledge Base — Adjusting belt tension (MK4/S, MK3.9/S, MK3.5/S, MK3/S)
• Prusa Knowledge Base — Adjusting belt tension (CORE One/+, CORE One INDX, CORE One L)
• Prusa Knowledge Base — Idler screw tension

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.