Trying to conceive can be stressful, and of course tech companies think that fertility gadgets can tell us the best time to have sex to get pregnant. But for most people, doing some simple math can provide the same answers, without spending any money.

In the sea of period trackers, two fertility gadgets that use slightly different approaches are the EarlySense Percept, a “contactless” device that you store under your mattress to monitor your fertility cycle, and the Mira AI monitor, which essentially combines hormonal information from multiple urine tests. (Both companies were at this year’s CES, and I visited both booths.)

The best time to conceive is right before a person ovulates, or releases an egg. Because internal body temperature fluctuates during the menstrual cycle, many people take their temperature each day to figure out where they are in the cycle. Products such as the Ava bracelet — and a variety of apps — try to make it easier to collect and interpret these readings. The selling point of the Percept, though, is that you can track temperature — and therefore, fertility — without actually needing to do anything. It’s is a slim white device that looks like a scale. Slide it under your mattress, and it will measure heart rate, breathing rate, and heart rate variability. That information is then fed to an algorithm. Because heart rate is correlated with temperature during the menstrual cycle, the founders say that its AI will tell you the best time to conceive, and keep making adjustments to get smarter. (They also say that it shouldn’t pick up information from your bed partner.)

Clearly, this seems simpler than actually taking your temperature each day. But, says Sara Vaughn, a reproductive health specialist and clinical fellow at Stanford University, temperature may not be the best way to track fertility in the first place.

Body temperature changes once someone has ovulated — and once that happens, it might already be too late to conceive. “Think of sperm as a really overconfident guy going on a date with a girl,” says Vaughn. “If he gets to the bar too soon, he’s gonna wait around a bit until the girl — the egg — gets there.” Sperm can hang around for about five days, waiting for the egg. But once you actually ovulate, you only have a 24-hour period to conceive, otherwise the egg leaves the body.

Tracking temperature can be helpful for people who have particularly long or abnormal menstrual cycles. “But if a patient has a normal cycle, I don’t need a special gadget to tell them when they should be having sex in the five days before the last 14th days in their cycle,” Vaughn says. It’s possible that a device could pick up heart rate and accurately gauge temperature through a mattress, but the basic method is not as good as ovulation prediction kits (OPK), or the “pee on a stick” method. These kits pick up a surge of hormone — called the luteinizing hormone (LH) — that increases right before someone ovulates, so these devices can be helpful about reminding people when to have sex.

The sleek, egg-shaped device Mira can combine information from multiple OPKs. Each day, you pee on a disposable test strip, and put it into the device. Then, Mira will read the levels of your luteinizing hormone and sync the data to an app that learns about your body’s cycle and tells you the best time to conceive. Because LH is released before ovulation, it’s a more useful measure, Vaughn says.

A device like Mira seems to be more precisely quantifying the LH in the urine, which is what doctors do when monitoring fertility through blood tests. But, again, for most people, this much information is still excessive — and expensive. (Mira will be available later this year for about $100 and the Percept is $199.) Regular periods are proof enough that someone is ovulating each month, and you just need some simple math to calculate when you’re most fertile. For example, say your cycles usually last 30 days and you’re going to have your next period on January 31st. You need to subtract 14 days to figure out the best time to have sex. So, that would be January 16th, the day you’re ovulating. The best times to conceive are the five days before and the one day after you ovulate, so from January 11th to the 17th.

If your periods are irregular, “you should see a doctor,” Vaughn says. “You shouldn’t do your own home monitoring to see if you’re ovulating, you should get checked out because the reason you’re not ovulating needs to be addressed.”

On average, adds Vaughn, fertile couples will be successful 20 percent of the time each month, so it’s normal for conception to take a few months. After a year, about 60 to 80 percent will have conceived. “And if you’ve been trying to have a baby for a year, once again, at that point you need to see a doctor,” she says. “None of these gadgets is going to figure out what is going on with you.”

Update January 17th, 6:30AM ET:* An earlier version of this article implied that a person’s period begins right after ovulation. Ovulation happens halfway through the cycle, and the period happens 14 days later.