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tokens: around 150 MHz 6 , the fluence limit at a 5 sigma level at zenith is 2.5 Jy ms for core stations or 1.5 Jy ms if all Dutch stations can be combined fully coherently. At that threshold, more than half of the known FRBs up to a DM of 500 pc cm −3 would be bright enough to be detected. This may be the most promising route to observe FRBs with LOFAR, since it would be possible to detect flat spectrum FRBs at 150 MHz. 6. Conclusions The FRATS project searches in real-time for millisecond-scale radio transients with the LOFAR radio telescope. Some known transients of this kind are FRBs and RRATs. The focus is to obtain an optimal sky coverage by using commensal observations and a large field of view. An innovative addition is the use of ring buffers that hold the last seconds of data from each individual antenna which ultimately allows for post-event beam-forming and imaging. This increases the positional accuracy, potentially to the full arc second resolution. This is important for pinpointing the location of FRBs. The potential of LOFAR is demonstrated by two different results. The first demonstration is the successful commensal observing from the Pilot C survey, where beamformed data were produced to search for millisecond-scale radio transients, during regular interferometric observations. Five pulsars were discovered in a 24-h scan. The second is the identification of a transient as a type II solar radio burst by localizing it in a side lobe. This could only be done by using the buffered data to create an all sky image. Besides these demonstrations we showed the results from the pilot surveys A and B, which searched up to a DM of ∼110 pc cm −3 and 500 pc cm −3 , respectively. While known pulsars were rediscovered, no unknown single-pulse sources have been detected. We have derived limits on the local FRB rates from these surveys. The first pilot survey (A) had an effective observing time of 68.51 beam h, observed from 119 to 151 MHz, the lowest frequency range on which FRBs have been searched. Taking into account the pointing direction and noise level for each observation, this sets a limit on the occurrence of FRBs of less than 1500 events per sky per day above a threshold of 1.6 kJy ms below a DM of ∼110 pc cm −3 . Pilot study B had an effective observing time of 91.2 beam h in the same band. This sets limits at 1400 events per sky per day or 134 events per Gpc 3 per day at a fluence threshold of 6.0 kJy ms for an 8-ms pulse. The expected rate is one per beam per 16 days, for a uniform extragalactic distribution, provided the source flux density follows a steep spectrum of ∼ ν −3 . In the case where the source population as a function of redshift follows the star formation rate, the rate drops to one per beam every 58 days. If the spectrum is less steep and/or a significant fraction of FRBs are broadened by scattering beyond our detection limits, a longer integration time is required. To be able to obtain the required observing time, we