Marinate a top round or flank steak for 4 to 24 hours, then sear it over direct high heat at about 450°F for 4 to 6 minutes per side until an instant-read thermometer reads 130 to 135°F in the thickest part. Rest the steak 5 to 10 minutes, and slice it thinly against the grain so each bite stays tender. A 1.5-pound piece of beef feeds four people for less than fifteen dollars when you handle it right.
This practical walkthrough covers everything from picking the right cut to slicing it thin, helping backyard grillers turn a budget-friendly London broil into a tender, crowd-pleasing dinner.
What a London Broil Actually Is
The name itself confuses almost every shopper the first time they pick up a package labeled “London broil” in the meat case. The name sounds like a cut, but it is really a preparation method and a label supermarkets slap on whichever lean roast fits the price point that week. Most often, the package in your hand is top round, a thick cut from the rear leg, or flank steak, a long flat muscle from the belly. Both are lean, both are flavorful, and both turn tough fast when mishandled.
Lean cuts carry very little intramuscular fat, the kind that melts and bastes a ribeye from the inside out. Top round runs about 4 grams of fat per 3-ounce serving, against 15 grams or more in a ribeye. That low-fat profile is exactly why London broil earns a chewy reputation on the grill, and exactly why a marinade, a hot fire, and a sharp knife matter so much.
How to Spot the Right Cut at the Store
Look for a piece labeled “London broil” that is roughly 1.25 to 1.5 inches thick and weighs 1.5 to 2 pounds. Top round usually appears as a rectangular roast with a deep red color and visible grain running lengthwise. Flank steak is thinner, longer, and shows pronounced fibers running down the piece. Either works on the grill, though flank cooks faster because of its lower thickness. When the butcher case has both, flank is more forgiving for first-timers; top round delivers a heartier, steakhouse-style plate when sliced thin.
With the right cut in hand, attention shifts to whether the marinade softens those fibers or just sits on the surface.
Choosing a Marinade That Tenderizes and Seasons
A good marinade does two jobs at once: it seasons the surface with salt, aromatics, and acid, and it begins to break down the tough muscle fibers that make lean beef so unforgiving. Acid (vinegar, citrus, or wine), oil, and salt form the backbone of nearly every marinade that works on London broil, and soy sauce pulls double duty as both salty seasoning and tenderizer thanks to its natural glutamates.
A reliable starting recipe: ½ cup soy sauce, ¼ cup olive oil, 3 tablespoons red wine vinegar, 1 tablespoon Worcestershire sauce, 4 minced garlic cloves, 1 teaspoon black pepper, and 1 teaspoon dried oregano. Whisk it in a zip-top bag, add the steak, press out the air, and refrigerate. That bag becomes your tenderizing chamber, and the meat absorbs flavor through every square inch of contact.
How Long to Marinate for Real Tenderness
Four hours is the minimum, and overnight (8 to 12 hours) is the sweet spot. The acid and salt need time to penetrate past the surface, and the enzymes in garlic and Worcestershire keep working for the full window. Marinating longer than 24 hours can turn the surface mushy, especially with flank steak, so set a timer and pull the bag at the 24-hour mark at the latest. If your week gets away from you, even 90 minutes at room temperature (around 70°F) improves both flavor and texture noticeably.
Quick Marinades When Time Is Short
Use a shallow pan instead of a bag, score the surface lightly in a crisscross pattern, and bump up the acid. A 30-minute soak in ½ cup balsamic vinegar mixed with 2 tablespoons olive oil, 1 tablespoon Dijon mustard, and a heavy pinch of salt gives you real penetration. Scoring opens channels for the liquid and helps the grill build extra sear in the cuts, which translates into more browned flavor.
Setting Up the Grill for Direct, High-Heat Cooking
London broil wants a hot, fast sear rather than a long, slow roast. Direct high heat drives the Maillard reaction on the surface, building that deep brown crust in minutes, while the interior stays juicy because the cook time is so short. Preheat a gas grill with all burners on high for 10 to 15 minutes, or build a charcoal fire and let the coals ash over until they glow orange and you can hold your hand 5 inches above the grate for only 3 to 4 seconds. Aim for a grate temperature around 450°F.
Lean beef plus a hot grill equals flare-ups, so set up two zones before you start. Push all the coals to one side of a charcoal grill, or leave one burner off on a gas grill. That cooler side becomes your safety valve: when flames leap up, slide the steak to the indirect zone for 30 seconds, let the flare die, and move it back. On a Weber kettle or any gas model with three or more burners, this two-zone setup is the difference between a confident sear and a charred exterior hiding raw meat in the middle.
That two-zone setup only pays off once you know what you’re actually aiming for on the inside.
Oil the grates lightly right before the steak goes on. Fold a paper towel in quarters, dip it in vegetable oil with tongs, and wipe it across the hot grates. A thin oil film keeps lean beef from sticking and tearing when you flip.
Grilling the Steak to the Right Internal Temperature
Pull the steak off the grill when an instant-read thermometer reads 130 to 135°F in the thickest part. That range delivers medium-rare, the doneness where lean beef stays juicy and tender. Anything past 145°F (medium) starts squeezing moisture out of the muscle fibers, and past 160°F (well-done), London broil turns into shoe leather. A Thermapen or any reliable instant-read thermometer removes the guesswork; aim for the 130 to 135°F window and you will hit the sweet spot almost every time.
Timing depends on thickness and grill temperature, but a 1.25-inch steak over direct 450°F heat usually needs about 4 to 6 minutes per side. Flank steak, which tends to run thinner, often finishes in 3 to 4 minutes per side. Always pull the steak off the grill 5°F below your target, because carryover cooking will lift the internal temperature another 5 to 8°F while it rests on the counter. That small adjustment is the difference between rosy medium-rare and gray, dry beef.
Using a Thermometer the Right Way
Insert the probe into the side of the steak, not the top, so the tip rests in the geometric center of the cut without touching bone or gristle. For a thick top round, slide the probe in from the long edge so it travels through the middle of the meat. Wait 5 seconds for the reading to stabilize. Clean the probe with hot soapy water between checks, and never leave the thermometer in the steak while it grills; the cable can melt and the metal skews the reading once it gets blazing hot.
Resting the Meat and Slicing Against the Grain
Resting is not optional, and slicing direction is where most home cooks undo an otherwise perfect steak. As soon as the London broil comes off the grill, transfer it to a cutting board and tent it loosely with foil. Let it rest 5 to 10 minutes, depending on thickness. During that window, the muscle fibers relax and reabsorb the juices that heat drove toward the center. Slice too early and those juices run straight out onto the board instead of staying in the meat.
Locating the grain takes 10 seconds and changes everything. Look closely at the surface of the cooked steak and you will see long lines of muscle fiber running in one direction, like the grain of oak. On flank steak, those lines are obvious. On top round, they run lengthwise down the roast. Rotate your knife 90 degrees to those lines, then slice. Each slice now crosses hundreds of shortened fibers instead of sliding between long ones, which is exactly what makes the bite feel tender.
Even with a sharp knife and a rested roast, small missteps earlier in the process can undo all that care.
Slice thin, about ¼ inch, against the grain. A sharp chef’s knife or long slicing knife works best; a serrated blade tears the fibers and shreds the crust. Angle the knife slightly and let it do the work instead of sawing.
Common Mistakes That Leave London Broil Tough or Dry
Most London broil disasters come down to four specific errors, and each one is easy to fix once you know what to look for. A marinade that ran under two hours, a grill that ran too cool, a steak that cooked past medium, or a knife that cut with the grain: any one of those will turn a $15 piece of beef into something you regret serving.
- Skipping the marinade: Without acid and salt breaking down the fibers, even a perfectly cooked steak chews like rubber. Two hours helps; 8 to 12 hours delivers the best result.
- Cooking past medium: Aim for 130 to 135°F and pull early. Lean beef has no fat buffer, so every degree past 140°F costs you moisture you cannot recover.
- Slicing with the grain: Find the lines and cut across them. Slicing parallel to the fibers leaves each piece as chewy as a rubber band.
- Cutting slices too thick: Half-inch slabs look impressive on the board and turn into jaw workouts on the plate. Stay near ¼ inch for tenderness.
- Skipping the rest: Cutting too early dumps the juices onto the cutting board. Five to ten minutes keeps them where they belong, inside the meat.
Quick Troubleshooting for Off Nights
When the steak came out gray on the outside, your grill was not hot enough; crank it next time and preheat longer. When the center was still cold after the timer ran out, your fire had a cold spot or the steak was thicker than you thought; move to the indirect zone to finish without burning the crust. When the slices felt tough even though the temperature was perfect, you cut with the grain; flip your knife 90 degrees and slice again. Most problems have a one-line fix once you can name them.
Final Thoughts
A great grilled London broil comes down to three levers: a long enough marinade to tenderize, a hot enough grill to sear, and a knife aimed across the grain. Get those right and a budget-friendly top round or flank steak eats like a much pricier cut. Aim for 130 to 135°F internal, rest the meat, slice thin, and you have a dependable backyard centerpiece for under twenty minutes of active cooking.
FAQ
What temperature should London broil be grilled at?
Set up a two-zone fire with direct high heat around 450°F, which sears the surface fast while giving you a cooler side to manage flare-ups. Pull the steak off when an instant-read thermometer reads 130 to 135°F for medium-rare, the doneness that keeps a lean cut juicy.
How long do you grill London broil on each side?
Plan on about 4 to 6 minutes per side for a 1.25-inch thick steak at high heat. Flank steak, which runs thinner, often finishes in 3 to 4 minutes per side. Rely on internal temperature rather than clock time, since grill intensity and steak thickness vary.
Do you need to marinate London broil before grilling?
Yes. A marinade with acid, oil, and salt tenderizes the fibers and seasons the meat. Four hours is the minimum, and 8 to 12 hours overnight delivers the best texture and flavor for top round or flank.
How do you know when London broil is done on the grill?
Use an instant-read thermometer in the thickest part of the steak. Remove it at 130 to 135°F for medium-rare, or 140 to 145°F for medium. The steak will rise another 5 to 8°F while it rests, so pull early.
Should London broil be cooked on high or low heat?
High heat. London broil is lean and needs a fast, hot cook to build a crust before the interior dries out. Low, slow heat overcooks the fibers and leaves you with a dry, gray piece of beef.
How do you make grilled London broil tender?
Marinate 4 to 12 hours, grill over direct 450°F heat to medium-rare, rest 5 to 10 minutes, and slice thinly against the grain. Skipping any of those steps is the usual reason a London broil turns out chewy.
