Hi, I have the Folger Reprap i3 print and I use Marlin frameware. This has bugged me for awhile now. When I load a gcode to Macpronterface, it gives me an estimate print time. However the actual print time is much longer, depends on how big the object is, sometimes it can be almost twice as long. Is the a way to fix this?

Try [gcode.ws]
It does a better job and probably takes acceleration and deceleration into account

Another reason why it may take longer is that Pronterface often cannot feed data over the USB port to a RAMPS board fast enough to keep up with the printer, especially if you have other tasks running on the computer at the same time. There is a patch you can do to Pronterface to make it faster by using hardware flow control instead of waiting for each command to be acknowledged, but it only works with electronics that has a native USB port, so not with Arduino Mega. However, if you try printing the file from SD card instead, you may find that the print time is much closer to the estimated time.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Could be a problem with pronterface.
I always use cura.
When printing via sd card its normall correct. given it takes a few minutes to heat.
Even if i print via cura and usb it normally within 10% of given time.
i.e. says it takes 30mins to print it might take 40mins and thats with a 6-8min heat time.