OligoCalc is an online tool for calculating the physical properties of DNA and RNA oligonucleotides, including molecular weight, melting temperature, absorbance, and %GC content. It provides a user-friendly interface where users can input nucleotide sequences and select options such as single- or double-stranded DNA/RNA, and various chemical modifications. The tool uses three methods for calculating melting temperature, including a nearest-neighbor thermodynamic model. OligoCalc has been widely used since its launch in 1997, with over 900,000 accesses from more than 200,000 distinct hosts. It is available at http://basic.northwestern.edu/biotools/OligoCalc.html and includes links to source code, usage statistics, and a table of chemical modifications. The tool also allows users to swap strands, perform BLAST searches, and calculate hairpin loop formation. OligoCalc is compatible with various web browsers and requires JavaScript to function. It is supported by Northwestern University and has received funding for development and operation. The tool is designed to help researchers compare melting temperatures using accurate calculations, with the nearest-neighbor method preferred for oligonucleotides between 8 and 40 nucleotides. For longer sequences or modified oligonucleotides, the salt-adjusted melting temperature calculation is preferred. OligoCalc is cited in several scientific references and has been used in various research applications.OligoCalc is an online tool for calculating the physical properties of DNA and RNA oligonucleotides, including molecular weight, melting temperature, absorbance, and %GC content. It provides a user-friendly interface where users can input nucleotide sequences and select options such as single- or double-stranded DNA/RNA, and various chemical modifications. The tool uses three methods for calculating melting temperature, including a nearest-neighbor thermodynamic model. OligoCalc has been widely used since its launch in 1997, with over 900,000 accesses from more than 200,000 distinct hosts. It is available at http://basic.northwestern.edu/biotools/OligoCalc.html and includes links to source code, usage statistics, and a table of chemical modifications. The tool also allows users to swap strands, perform BLAST searches, and calculate hairpin loop formation. OligoCalc is compatible with various web browsers and requires JavaScript to function. It is supported by Northwestern University and has received funding for development and operation. The tool is designed to help researchers compare melting temperatures using accurate calculations, with the nearest-neighbor method preferred for oligonucleotides between 8 and 40 nucleotides. For longer sequences or modified oligonucleotides, the salt-adjusted melting temperature calculation is preferred. OligoCalc is cited in several scientific references and has been used in various research applications.