Straight-line formula
Annual depreciation = (cost − salvage value) ÷ useful life. Monthly charge = annual ÷ 12. Book value = cost − annual depreciation × years elapsed, and it does not fall below salvage value.
Tax authorities in Pakistan, Saudi Arabia, and the United States often require their own rates or declining-balance methods. Use this for management accounts and budgets, then follow the method your auditor or tax filing uses.