Every measurement without regard on the used method and measuring device is burdened with smaller or bigger error. Because an error of the measurement is inevitable, it should be presented together with the result of the measurement. A simplified method of estimating of the uncertainty of measurement does not demand a lot of trials. It can be estimated by adding up the composition errors. One component of the uncertainty of measurement is an error of reading indications, which should be always to take into account when using analogous devices. By using digital devices this error does not step occur. The smaller the uncertainty of measurement is (however not always necessary), the higher price of digital devices in comparison with analogous devices.