Background and aims: Root biomass is one of the most widely used parameters to characterise root growth and belowground carbon stock. Our aim was to define a standard method to estimate the root biomass of young and adult oil palm trees in commercial plantations. Methods: Three methods based on the sampling excavation volume were compared using the same sampled tree. Work time and the number of workers required for each operation were recorded. We compared two large excavation volumes based on Voronoi tessellation and the standard root auger coring method in one 2-year-old and one 16-year-old commercial oil-palm plantation in Benin, West Africa. Results: Oil palm total root biomass was estimated at 0.84 and 22.23 Mg ha-1 in the 2-year-old and 16-year-old plantation, respectively. Compared to the reference method, the simplified Voronoi trench method estimated slightly higher (+5%) and lower (-17%) total root biomass with no significant differences but required 2 and 3 times more labour time, respectively, while the auger method estimated significantly lower (-23% and¿-¿53%) total root biomass in the 2-year-old and 16-year-old plantation, respectively. Coarse and fine root biomass were significantly higher under the windrow than under the footpath zones. Conclusion: The simplified Voronoi trench method required twice as much labour time as the auger method but was most efficient way to estimate oil palm total root biomass, irrespective of the age of the plantation.