diff --git a/packages/block-editor/src/components/grid-visualizer/grid-item-resizer.js b/packages/block-editor/src/components/grid-visualizer/grid-item-resizer.js
index 54683e48beeea4..33d677910a712e 100644
--- a/packages/block-editor/src/components/grid-visualizer/grid-item-resizer.js
+++ b/packages/block-editor/src/components/grid-visualizer/grid-item-resizer.js
@@ -45,33 +45,39 @@ export function GridItemResizer( { clientId, onChange } ) {
 					const rowGap = parseFloat(
 						getComputedCSS( gridElement, 'row-gap' )
 					);
-					const gridColumnLines = getGridLines(
+					const gridColumnTracks = getGridTracks(
 						getComputedCSS( gridElement, 'grid-template-columns' ),
 						columnGap
 					);
-					const gridRowLines = getGridLines(
+					const gridRowTracks = getGridTracks(
 						getComputedCSS( gridElement, 'grid-template-rows' ),
 						rowGap
 					);
-					const columnStart = getClosestLine(
-						gridColumnLines,
-						blockElement.offsetLeft
-					);
-					const rowStart = getClosestLine(
-						gridRowLines,
-						blockElement.offsetTop
-					);
-					const columnEnd = getClosestLine(
-						gridColumnLines,
-						blockElement.offsetLeft + boxElement.offsetWidth
-					);
-					const rowEnd = getClosestLine(
-						gridRowLines,
-						blockElement.offsetTop + boxElement.offsetHeight
-					);
+					const columnStart =
+						getClosestTrack(
+							gridColumnTracks,
+							blockElement.offsetLeft
+						) + 1;
+					const rowStart =
+						getClosestTrack(
+							gridRowTracks,
+							blockElement.offsetTop
+						) + 1;
+					const columnEnd =
+						getClosestTrack(
+							gridColumnTracks,
+							blockElement.offsetLeft + boxElement.offsetWidth,
+							'end'
+						) + 1;
+					const rowEnd =
+						getClosestTrack(
+							gridRowTracks,
+							blockElement.offsetTop + boxElement.offsetHeight,
+							'end'
+						) + 1;
 					onChange( {
-						columnSpan: Math.max( columnEnd - columnStart, 1 ),
-						rowSpan: Math.max( rowEnd - rowStart, 1 ),
+						columnSpan: columnEnd - columnStart + 1,
+						rowSpan: rowEnd - rowStart + 1,
 					} );
 				} }
 			/>
@@ -79,20 +85,50 @@ export function GridItemResizer( { clientId, onChange } ) {
 	);
 }
 
-function getGridLines( template, gap ) {
-	const lines = [ 0 ];
+/**
+ * Given a grid-template-columns or grid-template-rows CSS property value, gets the start and end
+ * position in pixels of each grid track.
+ *
+ * https://css-tricks.com/snippets/css/complete-guide-grid/#aa-grid-track
+ *
+ * @param {string} template The grid-template-columns or grid-template-rows CSS property value.
+ *                          Only supports fixed sizes in pixels.
+ * @param {number} gap      The gap between grid tracks in pixels.
+ *
+ * @return {Array<{start: number, end: number}>} An array of objects with the start and end
+ *                                               position in pixels of each grid track.
+ */
+function getGridTracks( template, gap ) {
+	const tracks = [];
 	for ( const size of template.split( ' ' ) ) {
-		const line = parseFloat( size );
-		lines.push( lines[ lines.length - 1 ] + line + gap );
+		const previousTrack = tracks[ tracks.length - 1 ];
+		const start = previousTrack ? previousTrack.end + gap : 0;
+		const end = start + parseFloat( size );
+		tracks.push( { start, end } );
 	}
-	return lines;
+	return tracks;
 }
 
-function getClosestLine( lines, position ) {
-	return lines.reduce(
-		( closest, line, index ) =>
-			Math.abs( line - position ) <
-			Math.abs( lines[ closest ] - position )
+/**
+ * Given an array of grid tracks and a position in pixels, gets the index of the closest track to
+ * that position.
+ *
+ * https://css-tricks.com/snippets/css/complete-guide-grid/#aa-grid-track
+ *
+ * @param {Array<{start: number, end: number}>} tracks   An array of objects with the start and end
+ *                                                       position in pixels of each grid track.
+ * @param {number}                              position The position in pixels.
+ * @param {string}                              edge     The edge of the track to compare the
+ *                                                       position to. Either 'start' or 'end'.
+ *
+ * @return {number} The index of the closest track to the position. 0-based, unlike CSS grid which
+ *                  is 1-based.
+ */
+function getClosestTrack( tracks, position, edge = 'start' ) {
+	return tracks.reduce(
+		( closest, track, index ) =>
+			Math.abs( track[ edge ] - position ) <
+			Math.abs( tracks[ closest ][ edge ] - position )
 				? index
 				: closest,
 		0