In the end, Seagraves advises against any online calculator. He favors spreadsheets.
"The most elementary spreadsheets that run on Android or iPhone devices can do a better job of calculating mortgage payments than the online calculators," he said.
Even the best calculators require accurate, current figures for taxes, insurance and other costs. Mark Burrage, executive director of mortgage digital experience for USAA in San Antonio, says the best results require figures specific to an individual house.
"Until you actually have a property, it's difficult to get down into all the costs," Burrage said. To get it all he suggests carefully scrutinizing the property listing and quizzing real estate agents, taxing authorities, insurance brokers and the current owner.
Calculators are useful for comparing interest rates, Burrage says, but they are rarely definitive and often give only a small picture of what the ultimate payment will be.
"They don't miscalculate," Seagraves said. "They just mislead."
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