The land area of a city proper divided by its population is one way to measure density. Here are some others, as posted in the Census Bureau thread:
Percentage of MSA living in Census tracts with population densities exceeding the national average (5,792 ppsm):
Los Angeles: 78.1%
San Jose: 75.0%
New York: 68.9%
San Francisco: 66.3%
Honolulu: 62.7%
Las Vegas: 62.0%
Miami: 57.4%
San Diego: 51.1%
Philadelphia: 46.8%
Chicago: 42.0%
Boston: 39.7%
Washington, DC: 38.8%
Seattle: 33.5%
Weighted population density, MSAs over 1 million, persons per square mile:
New York: 33,787.5
San Francisco....13,267.8
Honolulu....12,581.9
Los Angeles....12,169.4
San Jose....9,075.9
Chicago....9,011.9
Boston....8,987.9
Miami....8,489.2
Philadelphia....8,258.5
San Diego....7,381.9
Washington....7,296.1
Las Vegas....7,031.7
Seattle....6,146.3
Denver....5,418.0
Providence....5,204.6
Baltimore....5,144.7
Salt Lake City....5,070.9
Portland....5,058.8
Milwaukee....5,023.7
Sacramento....5,002.7
Phoenix....4,807.7
Riverside....4,636.9
Houston....4,606.4
New Orleans....4,577.0
Fresno....4,518.4
Buffalo....4,348.8
Dallas....4,274.7
Detroit....3,906.9
Minneapolis....3,784.4
Cleveland....3,676.9
Tampa Bay....3,616.6
Columbus....3,605.8
Virginia Beach....3,580.8
Austin....3,565.3
San Antonio....3,424.0
Tucson....3,285.2
Orlando....3,275.7
Hartford....3,195.3
Pittsburgh....2,970.0
Rochester....2,948.2
St. Louis....2,738.0
Atlanta....2,686.4
Louisville....2,686.3
Cincinnati....2,658.2
Oklahoma City....2,647.3
Richmond....2,590.4
Kansas City....2,561.4
Indianapolis....2,457.3
Jacksonville....2,431.3
Grand Rapids....2,413.3
Memphis....2,339.4
Tulsa....2,167.3
Raleigh....2,166.8
Charlotte....1,996.1
Nashville....1,943.3
Birmingham....1,402.6
Total MSA population of persons living in census tracts with densities of 20,000+ ppsm:
New York: 9,151,543
Los Angeles: 1,919,006
Chicago: 1,238,801
San Francisco: 899,765
Philadelphia: 841,729
Boston: 727,666
Washington: 501,510
Miami: 396,021
Honolulu: 182,167
Seattle: 160,101
San Diego: 103,421
Houston: 88,080
Baltimore: 67,095
San Jose: 64,724
Dallas: 54,893
Minneapolis: 52,998
Denver: 49,423
Milwaukee: 47,988
Bridgeport: 47,791
Providence: 39,442
Portland: 38,057
Madison: 35,514
Columbus: 31,592
Allentown: 29,319
Atlanta: 26,589
Worcester: 26,374
Las Vegas: 26,114
Austin: 23,224
Champaign: 22,271
Salinas: 21,893
Phoenix: 20,351
MSAs by % and total population living within census tracts with 10,000+ persons per square miles:
58.0% New York: 11,694,534
50.0% Los Angeles: 6,611,283
43.6% San Francisco: 2,073,127
38.8% Honolulu: 395,854
36.0% San Jose: 720,560
29.3% Boston: 1,448,764
27.1% Chicago: 2,614,012
26.4% Salinas: 116,532
25.3% Philadelphia: 1,580,169
24.7% San Diego: 816,530
23.2% Santa Barbara: 104,916
22.7% Miami: 1,398,475
19.5% Las Vegas: 441,510
19.2% Washington: 1,230,663
18.0% Oxnard: 152,811
18.0% Trenton: 70,272
17.9% Providence: 301,925
17.7% State College: 28,622
17.5% Bridgeport: 168,397
15.9% Santa Cruz: 43,412
14.3% Milwaukee: 226,941
14.2% Reading: 61,836
13.1% Baltimore: 375,152
13.1% Buffalo: 153,098
12.5% Seattle: 505,840
11.1% New Haven: 96,281
10.6% Denver: 315,809
9.9% Champaign: 22,271
9.7% Allentown: 84,293
9.2% Madison: 63,212
8.5% Manchester: 36,655
8.3% Lancaster: 46,505
8.3% Stockton: 65,403
8.0% Springfield, MA: 56,107
7.6% Atlantic City: 21,194
7.3% Riverside: 339,111
7.4% New Orleans: 95,502
7.1% Hartford: 87,780
7.1% Phoenix: 328,143
7.1% Portland: 179,612
6.9% Ann Arbor: 26,580
6.9% Houston: 495,906
6.8% Poughkeepsie: 48,699
6.7% Reno: 33,110
6.7% Worcester: 66,488
6.6% Boulder: 22,321
6.5% Minneapolis: 241,894
6.3% Albany: 57,733
6.3% Fresno: 64,225
6.2% Sacramento: 149,401
5.5% Rochester: 60,997
5.4% Harrisburg: 32,348
5.4% Scranton: 31,525
5.4% York: 25,122
5.1% Dallas: 390,927
4.7% Modesto: 26,829
4.6% Provo: 31,825
4.6% Vallejo: 21,593
4.0% Columbus: 86,536
4.0% Syracuse: 27,262
3.9% Bakersfield: 36,880
3.9% Pittsburgh: 94,694
3.7% Cleveland: 78,607
3.7% Salt Lake City: 47,020
3.3% Austin: 76,408
3.0% Richmond: 40,379
2.8% Detroit: 126,508
2.5% Orlando: 67,832
2.4% Atlanta: 150,542
2.0% Cincinnati: 46,615
1.8% St. Louis: 53,286
1.3% Tampa Bay: 43,634
1.2% San Antonio: 33,299
0.7% Charlotte: 21,929