Summary: The RF power deposited in the body during MRI, is generally measured by the specific absorption rate (SAR). Accurate knowledge of the SAR is essential for MRI safety and regulatory compliance. However we found that the MRI scanner's SAR estimates are often inaccurate and cannot be used to gauge actual RF exposure in any single experiment. Because commercial RF power monitoring equipment is unsuitable for measuring RF power over the large dynamic-range and duty cycles encountered during routine MRI, we designed, built and calibrated, a scanner-independent, multichannel real-time MRI RF power monitoring system. We used it to test the accuracy of scanner-reported SAR at 3T, finding reporting errors of up to 2.2-fold in commercial Philips and Siemens scanners. Further, we developed a RF SAR transducer that mimics the loading presented by the body, and used it, in combination with the RF power monitor, to create a 3T SAR dosimeter for independently monitoring RF exposure in Philips, GE, and Siemens MRI scanners.