In applications of NMR to biological macromolecules in solution, the concentration of the NMR sample is an important parameter describing the sample and providing information for the selection and planning of experiments. Although concentrations can be measured directly by NMR spectroscopy, other methods are usually preferred to measure the concentration of macromolecules in NMR samples. The reasons are the difficulties in the correlation of the sample of interest with the signal intensity representing a known concentration. This correlation is usually obtained by adding to the sample a reference compound with known concentration and comparing the integral over resolved resonance lines of the molecules with known and unknown concentrations. For solutions of biological macromolecules it is very difficult to find a compound that does not interact with the macromolecules and has a resonance outside their spectral range. We introduce PULCON which is a method that correlates the absolute intensities of two spectra measured in different solution conditions. PULCON is easy to implement and apply on all NMR spectrometers; it does not need any special hardware or software. PULCON is very robust and at the same time delivers accurate concentrations of samples in the NMR tube. We demonstrate that PULCON has the potential to replace UV spectroscopy for concentration measurements of NMR samples.