The maths behind every number

Rates and their dates

Market figures such as gold, the dollar, KIBOR and the policy rate are fetched each day from their source and checked before they go live. A sudden or out-of-range move waits for a person to confirm it. Bank terms and electricity tariffs are watched for changes and updated by hand.

Loan instalments

Home, car and personal loan calculators use the standard reducing-balance formula that Pakistani banks quote:

Monthly instalment = P × r × (1 + r)ⁿ ÷ ((1 + r)ⁿ − 1)

P is the amount borrowed, r is the yearly rate divided by 12, and n is the number of months. Interest each month is the remaining balance times r. The rest of the instalment reduces the balance. Extra payments go straight to the balance, which shortens the loan.

Banks may add processing fees, insurance or takaful, and Islamic products describe the profit differently. These are not included unless the calculator asks for them.

Apna Ghar subsidy

The Apna Ghar calculator follows the Wazir-e-Azam Apna Ghar Program terms in SBP SH&SFD Circular 03/2025. For the first 10 years the instalment is worked out at the subsidised rate over the full tenure. At the start of year 11, the remaining balance is re-spread over the months left at the post-subsidy rate, which the program sets at 1-year KIBOR + 3%.

Nobody knows what KIBOR will be in ten years, so the post-subsidy rate is an input you can change. Try a higher rate to see the risk.

Zakat

Zakat is 2.5% of zakatable wealth (cash, gold, silver, investments and money owed to you, minus debts due now) when that wealth is above the nisab. The nisab is the value of 52.5 tola of silver or 7.5 tola of gold. The calculator uses the silver nisab by default because it is lower, which is the more cautious view. Ask a scholar you trust if you are unsure which applies to you.

Salary tax

Annual taxable salary is matched to its slab. Tax is the fixed amount for that slab plus the slab rate on income above its lower limit. Income up to Rs 600,000 a year is not taxed. Monthly tax is the annual figure divided by 12. Surcharges, allowances and rebates are not included.

Construction cost

Cost is covered area times a per-sq-ft rate for the grey structure and for finishing, chosen by quality tier. About 81% of the grey structure is treated as materials and the rest as labour. Cement and steel quantities use about 0.47 bags and 2.7 kg per sq ft. Rates vary a lot by city, contractor and month, so treat the result as a starting point for quotes.

Solar

Each kW of panels is taken to make peak sun hours × 0.78 units a day. The 22% loss covers heat, dust, wiring and the inverter. Sun hours are yearly averages for each city, and output falls by 0.5% a year as panels age.

For payback, units you use yourself save your grid rate, which rises by the yearly percentage you choose. Under net billing, exported units earn the export rate. Under older 1:1 net metering, they earn the grid rate. Payback is the point where total savings first equal the system cost.

Battery size is running watts × backup hours, plus 10% for inverter losses, divided by how deeply the battery can be drained (90% for lithium, 50% for tubular). Inverter size is the larger of running load × 1.25 and the start-up peak ÷ 2, rounded up to a standard size.

Bank solar calculators use only terms published on each bank’s own website, with the date we checked them. Where a bank does not publish its rate, you enter the rate from your quotation.

Panel angles use Charles Landau’s fits for fixed panels facing the equator, for example latitude × 0.76 + 3.1 for the best year-round angle between 25° and 50°.

Land and units

One marla is taken as 225 sq ft and one kanal as 20 marla, which is what most banks and housing schemes use today. Some older records use 272.25 sq ft per marla. Check which one your documents use.

What we leave out

• Bank-specific fees, insurance and late-payment charges.
• Changes to rates after you take a loan, unless you enter them.
• Personal circumstances that a bank or tax adviser would consider.

If a result does not match what your bank quoted, the difference is usually one of these. Tell us if you think our maths is wrong.