Skills Development Levy (SDL) is a compulsory levy that employers have to pay for all employees working in Singapore, on top of CPF contribution and Foreign Worker Levy.

"All employees" includes Singapore Citizens, Singapore Permanent Residents and Foreigners of all Singapore work pass types, including full-time, casual, part-time, temporary type of contracts, except the exempted employee categories.

The exempted employee categories are defined here under the FAQ question: Which categories of employees or workers are exempted from SDL?
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An employer is required to pay the SDL monthly even for those on a casual, part-time, or temporary basis.
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The SDL payable is at 0.25% of an employee's monthly total wages at a minimum of $2 (less than $800 earned a month) and a maximum of $11.25 (if total wages earned per month is more than $4,500).

You can pay SDL together with your CPF contributions. Refer to this link for more information: https://www.cpf.gov.sg/employer/employer-obligations/skills-development-levy