Advisers can use the tool to check clients’ eligibility and criteria before submitting a case.
The lender said it had developed the calculator in response to broker feedback.
It comes after new Prudential Regulation Authority (PRA) rules introduced last year led to some confusion over lenders’ varied approaches.
Phil Rickards, head of BM Solutions (pictured), said: “After an early review, we have simplified our portfolio process with the introduction of a new online portfolio calculator to help brokers to calculate LTV and Rental Cover Ratios for portfolio lending.
“We have responded swiftly to broker feedback by enhancing our customer profile form – making it easier to complete, and enabling brokers to check the key elements of portfolio criteria prior to submitting the case.
“We have demonstrated our commitment to supporting intermediaries as the market continues to adapt to these changes, by reaching more than 4000 brokers through our nationwide workshops and continuing to back up our five-star service by making improvements designed around brokers’ needs.”
BM Solutions limit is 10 buy-to-let mortgaged properties and a minimum earned income of £30,000 is also required.
The maximum aggregate portfolio is 75% LTV and the minimum aggregate portfolio rental cover ratio is 145% stressed at 5.5%.