About this tool
The DNA / Oligo Molecular Weight Calculator is a free, in-browser tool that estimates the molecular weight of a nucleic-acid sequence from its base composition. Paste a DNA or RNA sequence, choose single-stranded DNA, double-stranded DNA, or RNA, and it returns the molecular weight in g/mol along with the length, GC content, and a nanomole-to-microgram conversion. Sequences are processed locally in your browser and never uploaded.
The weight is the sum of anhydrous nucleotide monophosphate masses minus a 61.96 g/mol correction for the removal of a phosphate at the 5′ end. DNA bases use dA 313.21, dT 304.2, dC 289.18, dG 329.21; RNA uses A 329.21, U 306.17, C 305.18, G 345.21. For double-stranded DNA the tool adds the weight of the complementary strand (A↔T, C↔G) so dsDNA ≈ 2× the single strand.
Amount conversions use µg = nmol × MW ÷ 1000, and the tool also reports µg per nmol (MW ÷ 1000). This gives an approximate MW suitable for ordering and resuspending oligos; it counts only A, T/U, G, and C, so ambiguity codes or modifications are not included.