Vegetation ecologists use a wide range of importance (“abundance”) measures for plant species in vegetation plots, including percentage cover, ordinal cover scales, frequency and fractional biomass. Resurvey studies and analyses of heterogenous data from large vegetation-plot databases need to deal with data sampled with different approaches, yet very little is known about how these approaches relate to each other. For the Square Foot Project, in which hundreds of historic grassland plots in Switzerland of 0.09 m2 size were resurveyed after having been originally sampled via biomass harvest, we had the challenge that the historical method was too time-consuming for our study and thus had to resort to careful cover estimation in percent. However, we used both methods to sample 40 such plots representative of the grasslands of Switzerland and compared the results. We found that with biomassbased species determination, an average of 0.9 additional species (4.6%) per plot were found compared to cover-based sampling. Graminoids were three times more likely to be overlooked than forbs. Fractional cover was well related to fractional biomass with an allometric (power-law) function, with an exponent of 0.6 for all species combined, while graminoids and forbs showed clear differences in function parameters when analysed separately. Generally, our overlooking probabilities were much lower than in other comparable studies, which might be due to the very small plot size or to the careful sampling with percentage estimates. Our allometric functions allow reliable transformations between fractional biomass and fractional cover in temperate European grasslands. We recommend the development of further such empirical functions for other regions and ecosystems.